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CSharpGuidelines

C# coding guidelines (one page version ofhttps://www.csharpcodingguidelines.com)

C# coding guidelines

Single page version of https://csharpcodingguidelines.com/ .

Class Design Guidelines

AV1000: A class or interface should have a single purpose

A class or interface should have a single purpose within the system it functions in. In general, a class either represents a primitive type like an email or ISBN number, an abstraction of some business concept, a plain data structure, or is responsible for orchestrating the interaction between other classes. It is never a combination of those. This rule is widely known as the Single Responsibility Principle, one of the S.O.L.I.D. principles.

Tip: A class with the word And in it is an obvious violation of this rule.

Tip: Use Design Patterns to communicate the intent of a class. If you can’t assign a single design pattern to a class, chances are that it is doing more than one thing.

Note If you create a class representing a primitive type you can greatly simplify its use by making it immutable.

AV1001: Only create a constructor that returns a useful object

There should be no need to set additional properties before the object can be used for whatever purpose it was designed. However, if your constructor needs more than three parameters (which violates AV1561), your class might have too much responsibility (and violates AV1000).

AV1003: An interface should be small and focused

Interfaces should have a name that clearly explains their purpose or role in the system. Do not combine many vaguely related members on the same interface just because they were all on the same class. Separate the members based on the responsibility of those members, so that callers only need to call or implement the interface related to a particular task. This rule is more commonly known as the Interface Segregation Principle.

AV1004: Use an interface rather than a base class to support multiple implementations

If you want to expose an extension point from your class, expose it as an interface rather than as a base class. You don’t want to force users of that extension point to derive their implementations from a base class that might have an undesired behavior. However, for their convenience you may implement a(n abstract) default implementation that can serve as a starting point.

AV1005: Use an interface to decouple classes from each other

Interfaces are a very effective mechanism for decoupling classes from each other:

AV1008: Avoid static classes

With the exception of extension method containers, static classes very often lead to badly designed code. They are also very difficult, if not impossible, to test in isolation, unless you’re willing to use some very hacky tools.

Note: If you really need that static class, mark it as static so that the compiler can prevent instance members and instantiating your class. This relieves you of creating an explicit private constructor.

AV1010: Don’t suppress compiler warnings using the new keyword

Compiler warning CS0114 is issued when breaking Polymorphism, one of the most essential object-orientation principles. The warning goes away when you add the new keyword, but it keeps sub-classes difficult to understand. Consider the following two classes:

public class Book  
{
    public virtual void Print()  
    {
        Console.WriteLine("Printing Book");
    }  
}

public class PocketBook : Book  
{
    public new void Print()
    {
        Console.WriteLine("Printing PocketBook");
    }  
}

This will cause behavior that you would not normally expect from class hierarchies:

PocketBook pocketBook = new PocketBook();

pocketBook.Print(); // Outputs "Printing PocketBook "

((Book)pocketBook).Print(); // Outputs "Printing Book"

It should not make a difference whether you call Print() through a reference to the base class or through the derived class.

AV1011: It should be possible to treat a derived type as if it were a base type

In other words, you should be able to pass an instance of a derived class wherever its base class is expected, without the callee knowing the derived class. A very notorious example of a violation of this rule is throwing a NotImplementedException when overriding methods from a base class. A less subtle example is not honoring the behavior expected by the base class.

Note: This rule is also known as the Liskov Substitution Principle, one of the S.O.L.I.D. principles.

AV1013: Don’t refer to derived classes from the base class

Having dependencies from a base class to its sub-classes goes against proper object-oriented design and might prevent other developers from adding new derived classes.

AV1014: Avoid exposing the other objects an object depends on

If you find yourself writing code like this then you might be violating the Law of Demeter.

someObject.SomeProperty.GetChild().Foo()

An object should not expose any other classes it depends on because callers may misuse that exposed property or method to access the object behind it. By doing so, you allow calling code to become coupled to the class you are using, and thereby limiting the chance that you can easily replace it in a future stage.

Note: Using a class that is designed using the Fluent Interface pattern seems to violate this rule, but it is simply returning itself so that method chaining is allowed.

Exception: Inversion of Control or Dependency Injection frameworks often require you to expose a dependency as a public property. As long as this property is not used for anything other than dependency injection I would not consider it a violation.

AV1020: Avoid bidirectional dependencies

This means that two classes know about each other’s public members or rely on each other’s internal behavior. Refactoring or replacing one of those classes requires changes on both parties and may involve a lot of unexpected work. The most obvious way of breaking that dependency is to introduce an interface for one of the classes and using Dependency Injection.

Exception: Domain models such as defined in Domain-Driven Design tend to occasionally involve bidirectional associations that model real-life associations. In those cases, make sure they are really necessary, and if they are, keep them in.

AV1025: Classes should have state and behavior

In general, if you find a lot of data-only classes in your code base, you probably also have a few (static) classes with a lot of behavior (see AV1008). Use the principles of object-orientation explained in this section and move the logic close to the data it applies to.

Exception: The only exceptions to this rule are classes that are used to transfer data over a communication channel, also called Data Transfer Objects, or a class that wraps several parameters of a method.

AV1026: Classes should protect the consistency of their internal state

Validate incoming arguments from public members. For example:

public void SetAge(int years)
{
    AssertValueIsInRange(years, 0, 200, nameof(years));

    this.age = years;
}

Protect invariants on internal state. For example:

public void Render()
{
    AssertNotDisposed();

    // ...
}

Member Design Guidelines

AV1100: Allow properties to be set in any order

Properties should be stateless with respect to other properties, i.e. there should not be a difference between first setting property DataSource and then DataMember or vice-versa.

AV1105: Use a method instead of a property

Exception: Populating an internal cache or implementing lazy-loading is a good exception.

AV1110: Don’t use mutually exclusive properties

Having properties that cannot be used at the same time typically signals a type that represents two conflicting concepts. Even though those concepts may share some of their behavior and states, they obviously have different rules that do not cooperate.

This violation is often seen in domain models and introduces all kinds of conditional logic related to those conflicting rules, causing a ripple effect that significantly increases the maintenance burden.

AV1115: A property, method or local function should do only one thing

Similarly to rule AV1000, a method body should have a single responsibility.

AV1125: Don’t expose stateful objects through static members

A stateful object is an object that contains many properties and lots of behavior behind it. If you expose such an object through a static property or method of some other object, it will be very difficult to refactor or unit test a class that relies on such a stateful object. In general, introducing a construct like that is a great example of violating many of the guidelines of this chapter.

A classic example of this is the HttpContext.Current property, part of ASP.NET. Many see the HttpContext class as a source of a lot of ugly code.

AV1130: Return interfaces to unchangeable collections

You generally don’t want callers to be able to change an internal collection, so don’t return arrays, lists or other collection classes directly. Instead, return an IEnumerable<T>, IAsyncEnumerable<T>, IQueryable<T>, IReadOnlyCollection<T>, IReadOnlyList<T>, IReadOnlySet<T> or IReadOnlyDictionary<TKey, TValue>.

Exception: Immutable collections such as ImmutableArray<T>, ImmutableList<T> and ImmutableDictionary<TKey, TValue> prevent modifications from the outside and are thus allowed.

AV1135: Properties, arguments and return values representing strings, collections or tasks should never be null

Returning null can be unexpected by the caller. Always return an empty collection or an empty string instead of a null reference. When your member returns Task or Task<T>, return Task.CompletedTask or Task.FromResult(). This also prevents cluttering your code base with additional checks for null, or even worse, string.IsNullOrEmpty().

AV1137: Define parameters as specific as possible

If your method or local function needs a specific piece of data, define parameters as specific as that and don’t take a container object instead. For instance, consider a method that needs a connection string that is exposed through a central IConfiguration interface. Rather than taking a dependency on the entire configuration, just define a parameter for the connection string. This not only prevents unnecessary coupling, it also improves maintainability in the long run.

Note: An easy trick to remember this guideline is the Don’t ship the truck if you only need a package.

AV1140: Consider using domain-specific value types rather than primitives

Instead of using strings, integers and decimals for representing domain-specific types such as an ISBN number, an email address or amount of money, consider creating dedicated value objects that wrap both the data and the validation rules that apply to it. By doing this, you prevent ending up having multiple implementations of the same business rules, which both improves maintainability and prevents bugs.

Miscellaneous Design Guidelines

AV1200: Throw exceptions rather than returning some kind of status value

A code base that uses return values to report success or failure tends to have nested if-statements sprinkled all over the code. Quite often, a caller forgets to check the return value anyway. Structured exception handling has been introduced to allow you to throw exceptions and catch or replace them at a higher layer. In most systems it is quite common to throw exceptions whenever an unexpected situation occurs.

AV1202: Provide a rich and meaningful exception message text

The message should explain the cause of the exception, and clearly describe what needs to be done to avoid the exception.

AV1205: Throw the most specific exception that is appropriate

For example, if a method receives a null argument, it should throw ArgumentNullException instead of its base type ArgumentException.

AV1210: Don’t swallow errors by catching generic exceptions

Avoid swallowing errors by catching non-specific exceptions, such as Exception, SystemException, and so on, in application code. Only in top-level code, such as a last-chance exception handler, you should catch a non-specific exception for logging purposes and a graceful shutdown of the application.

AV1215: Properly handle exceptions in asynchronous code

When throwing or handling exceptions in code that uses async/await or a Task remember the following two rules:

AV1220: Always check an event handler delegate for null

An event that has no subscribers is null. So before invoking, always make sure that the delegate list represented by the event variable is not null. Invoke using the null conditional operator, because it additionally prevents conflicting changes to the delegate list from concurrent threads.

event EventHandler<NotifyEventArgs> Notify;

protected virtual void OnNotify(NotifyEventArgs args)
{
    Notify?.Invoke(this, args);
}

AV1235: Don’t pass null as the sender argument when raising an event

Often an event handler is used to handle similar events from multiple senders. The sender argument is then used to get to the source of the event. Always pass a reference to the source (typically this) when raising the event. Furthermore don’t pass null as the event data parameter when raising an event. If there is no event data, pass EventArgs.Empty instead of null.

Exception: On static events, the sender argument should be null.

AV1240: Use generic constraints if applicable

Instead of casting to and from the object type in generic types or methods, use where constraints or the as operator to specify the exact characteristics of the generic parameter. For example:

class SomeClass  
{

}

// Don't  
class MyClass  
{
    void SomeMethod(T t)  
    {  
        object temp = t;
        SomeClass obj = (SomeClass) temp;
    }  
}

// Do  
class MyClass where T : SomeClass  
{
    void SomeMethod(T t)  
    {  
        SomeClass obj = t;
    }  
}

AV1250: Evaluate the result of a LINQ expression before returning it

Consider the following code snippet

public IEnumerable<GoldMember> GetGoldMemberCustomers()
{
    const decimal GoldMemberThresholdInEuro = 1_000_000;

    var query =
        from customer in db.Customers
        where customer.Balance > GoldMemberThresholdInEuro
        select new GoldMember(customer.Name, customer.Balance);

    return query;
}

Since LINQ queries use deferred execution, returning query will actually return the expression tree representing the above query. Each time the caller evaluates this result using a foreach loop or similar, the entire query is re-executed resulting in new instances of GoldMember every time. Consequently, you cannot use the == operator to compare multiple GoldMember instances. Instead, always explicitly evaluate the result of a LINQ query using ToList(), ToArray() or similar methods.

AV1251: Do not use this and base prefixes unless it is required

In a class hierarchy, it is not necessary to know at which level a member is declared to use it. Refactoring derived classes is harder if that level is fixed in the code.

Maintainability Guidelines

AV1500: Methods should not exceed 7 statements

A method that requires more than 7 statements is simply doing too much or has too many responsibilities. It also requires the human mind to analyze the exact statements to understand what the code is doing. Break it down into multiple small and focused methods with self-explaining names, but make sure the high-level algorithm is still clear.

AV1501: Make all members private and types internal sealed by default

To make a more conscious decision on which members to make available to other classes, first restrict the scope as much as possible. Then carefully decide what to expose as a public member or type.

AV1502: Avoid conditions with double negatives

Although a property like customer.HasNoOrders makes sense, avoid using it in a negative condition like this:

bool hasOrders = !customer.HasNoOrders;

Double negatives are more difficult to grasp than simple expressions, and people tend to read over the double negative easily.

AV1505: Name assemblies after their contained namespace

All DLLs should be named according to the pattern Company.Component.dll where Company refers to your company’s name and Component contains one or more dot-separated clauses. For example AvivaSolutions.Web.Controls.dll.

As an example, consider a group of classes organized under the namespace AvivaSolutions.Web.Binding exposed by a certain assembly. According to this guideline, that assembly should be called AvivaSolutions.Web.Binding.dll.

Exception: If you decide to combine classes from multiple unrelated namespaces into one assembly, consider suffixing the assembly name with Core, but do not use that suffix in the namespaces. For instance, AvivaSolutions.Consulting.Core.dll.

AV1506: Name a source file to the type it contains

Use Pascal casing to name the file and don’t use underscores. Don’t include (the number of) generic type parameters in the file name.

AV1507: Limit the contents of a source code file to one type

Exception: Nested types should be part of the same file.

Exception: Types that only differ by their number of generic type parameters should be part of the same file.

AV1508: Name a source file to the logical function of the partial type

When using partial types and allocating a part per file, name each file after the logical part that part plays. For example:

// In MyClass.cs
public partial class MyClass
{...}

// In MyClass.Designer.cs
public partial class MyClass
{...}

AV1510: Use using statements instead of fully qualified type names

Limit usage of fully qualified type names to prevent name clashing. For example, don’t do this:

var list = new System.Collections.Generic.List<string>();

Instead, do this:

using System.Collections.Generic;

var list = new List<string>();

If you do need to prevent name clashing, use a using directive to assign an alias:

using Label = System.Web.UI.WebControls.Label;

AV1515: Don’t use “magic” numbers

Don’t use literal values, either numeric or strings, in your code, other than to define symbolic constants. For example:

public class Whatever  
{
    public static readonly Color PapayaWhip = new Color(0xFFEFD5);
    public const int MaxNumberOfWheels = 18;
    public const byte ReadCreateOverwriteMask = 0b0010_1100;
}

Strings intended for logging or tracing are exempt from this rule. Literals are allowed when their meaning is clear from the context, and not subject to future changes, For example:

mean = (a + b) / 2; // okay  
WaitMilliseconds(waitTimeInSeconds * 1000); // clear enough

If the value of one constant depends on the value of another, attempt to make this explicit in the code.

public class SomeSpecialContainer  
{  
    public const int MaxItems = 32;
    public const int HighWaterMark = 3 * MaxItems / 4;// at 75%  
}

Note: An enumeration can often be used for certain types of symbolic constants.

AV1520: Only use var when the type is evident

Use var for anonymous types (typically resulting from a LINQ query), or if the type is evident. Never use var for built-in types.

// Projection into anonymous type.
var largeOrders =
    from order in dbContext.Orders
    where order.Items.Count > 10 && order.TotalAmount > 1000
    select new { order.Id, order.TotalAmount };

// Built-in types.
bool isValid = true;
string phoneNumber = "(unavailable)";
uint pageSize = Math.Max(itemCount, MaxPageSize);  

// Types are evident.
var customer = new Customer();
var invoice = Invoice.Create(customer.Id);
var user = sessionCache.Resolve<User>("john.doe@mail.com");
var subscribers = new List<Subscriber>();
var summary = shoppingBasket.ToOrderSummary(); 

// All other cases.
IQueryable<Order> recentOrders = ApplyFilter(order => order.CreatedAt > DateTime.Now.AddDays(-30));
LoggerMessage message = Compose(context);
ReadOnlySpan<char> key = ExtractKeyFromPair("email=john.doe@mail.com");
IDictionary<Category, Product> productsPerCategory = shoppingBasket.Products.ToDictionary(product => product.Category);

AV1521: Declare and initialize variables as late as possible

Avoid the C and Visual Basic styles where all variables have to be defined at the beginning of a block, but rather define and initialize each variable at the point where it is needed.

AV1522: Assign each variable in a separate statement

Don’t use confusing constructs like the one below:

var result = someField = GetSomeMethod();

Exception: Multiple assignments per statement are allowed by using out variables, is-patterns or deconstruction into tuples. Examples:

bool success = int.TryParse(text, out int result);

if ((items[0] is string text) || (items[1] is Action action))
{
}

(string name, string value) = SplitNameValuePair(text);

AV1523: Favor object and collection initializers over separate statements

Instead of:

var startInfo = new ProcessStartInfo("myapp.exe");
startInfo.StandardOutput = Console.Output;
startInfo.UseShellExecute = true;

var countries = new List();
countries.Add("Netherlands");
countries.Add("United States");

var countryLookupTable = new Dictionary<string, string>();
countryLookupTable.Add("NL", "Netherlands");
countryLookupTable.Add("US", "United States");

Use Object and Collection Initializers:

var startInfo = new ProcessStartInfo("myapp.exe")  
{
    StandardOutput = Console.Output,
    UseShellExecute = true  
};

var countries = new List { "Netherlands", "United States" };

var countryLookupTable = new Dictionary<string, string>
{
    ["NL"] = "Netherlands",
    ["US"] = "United States"
};

AV1525: Don’t make explicit comparisons to true or false

It is usually bad style to compare a bool-type expression to true or false. For example:

while (condition == false) // wrong; bad style  
while (condition != true) // also wrong  
while (((condition == true) == true) == true) // where do you stop?  
while (condition) // OK

AV1530: Don’t change a loop variable inside a for loop

Updating the loop variable within the loop body is generally considered confusing, even more so if the loop variable is modified in more than one place.

for (int index = 0; index < 10; ++index)  
{  
    if (someCondition)
    {
        index = 11; // Wrong! Use 'break' or 'continue' instead.  
    }
}

AV1532: Avoid nested loops

A method that nests loops is more difficult to understand than one with only a single loop. In fact, in most cases nested loops can be replaced with a much simpler LINQ query that uses the from keyword twice or more to join the data.

DG1535: Always add a block after the keywords if, else, do, while, for, foreach and case

Please note that this also avoids possible confusion in statements of the form:

if (isActive) if (isVisible) Foo(); else Bar(); // which 'if' goes with the 'else'?

// The right way:  
if (isActive)  
{  
    if (isVisible)  
    {  
        Foo()
    }  
    else  
    {  
        Bar();
    }  
}

Note that a block after the keyword using is also expected, but can be omitted when two usings are used successively.

Exception: case statements that only call return.

AV1536: Always add a default block after the last case in a switch statement

Add a descriptive comment if the default block is supposed to be empty. Moreover, if that block is not supposed to be reached throw an InvalidOperationException to detect future changes that may fall through the existing cases. This ensures better code, because all paths the code can travel have been thought about.

void Foo(string answer)  
{  
    switch (answer)  
    {  
        case "no":  
        {
            Console.WriteLine("You answered with No");
            break;
        }  

        case "yes":
        {  
            Console.WriteLine("You answered with Yes");
            break;
        }

        default:  
        {
            // Not supposed to end up here.  
            throw new InvalidOperationException("Unexpected answer " + answer);
        }  
    }  
}

AV1537: Finish every if-else-if statement with an else clause

For example:

void Foo(string answer)  
{  
    if (answer == "no")  
    {  
        Console.WriteLine("You answered with No");
    }  
    else if (answer == "yes")  
    {  
        Console.WriteLine("You answered with Yes");
    }  
    else  
    {  
        // What should happen when this point is reached? Ignored? If not,
        // throw an InvalidOperationException.  
    }  
}

AV1540: Be reluctant with multiple return statements

One entry, one exit is a sound principle and keeps control flow readable. However, if the method body is very small and complies with guideline AV1500 then multiple return statements may actually improve readability over some central boolean flag that is updated at various points.

AV1545: Don’t use an if-else construct instead of a simple (conditional) assignment

Express your intentions directly. For example, rather than:

bool isPositive;

if (value > 0)
{
    isPositive = true;
}
else
{
    isPositive = false;
}

write:

bool isPositive = value > 0;

Or instead of:

string classification;

if (value > 0)
{
    classification = "positive";
}
else
{
    classification = "negative";
}

return classification;

write:

return value > 0 ? "positive" : "negative";

Or instead of:

int result;

if (offset == null)
{
    result = -1;
}
else
{
    result = offset.Value;
}

return result;

write:

return offset ?? -1;

Or instead of:

private DateTime? firstJobStartedAt;

public void RunJob()
{
    if (firstJobStartedAt == null)
    {
        firstJobStartedAt = DateTime.UtcNow;
    }
}

write:

private DateTime? firstJobStartedAt;

public void RunJob()
{
    firstJobStartedAt ??= DateTime.UtcNow;
}

Or instead of:

if (employee.Manager != null)
{
    return employee.Manager.Name;
}
else
{
    return null;
}

write:

return employee.Manager?.Name;

AV1546: Prefer interpolated strings over concatenation or string.Format

Since .NET 6, interpolated strings are optimized at compile-time, which inlines constants and reduces memory allocations due to boxing and string copying.

// GOOD
string result = $"Welcome, {firstName} {lastName}!";

// BAD
string result = string.Format("Welcome, {0} {1}!", firstName, lastName);

// BAD
string result = "Welcome, " + firstName + " " + lastName + "!";

// BAD
string result = string.Concat("Welcome, ", firstName, " ", lastName, "!");

AV1547: Encapsulate complex expressions in a property, method or local function

Consider the following example:

if (member.HidesBaseClassMember && member.NodeType != NodeType.InstanceInitializer)
{
    // do something
}

In order to understand what this expression is about, you need to analyze its exact details and all of its possible outcomes. Obviously, you can add an explanatory comment on top of it, but it is much better to replace this complex expression with a clearly named method:

if (NonConstructorMemberUsesNewKeyword(member))  
{  
    // do something
}  


private bool NonConstructorMemberUsesNewKeyword(Member member)  
{  
    return member.HidesBaseClassMember &&
           member.NodeType != NodeType.InstanceInitializer;
}

You still need to understand the expression if you are modifying it, but the calling code is now much easier to grasp.

AV1551: Call the more overloaded method from other overloads

This guideline only applies to overloads that are intended to provide optional arguments. Consider, for example, the following code snippet:

public class MyString  
{
    private string someText;

    public int IndexOf(string phrase)  
    {  
        return IndexOf(phrase, 0);
    }

    public int IndexOf(string phrase, int startIndex)  
    {  
        return IndexOf(phrase, startIndex, someText.Length - startIndex);
    }

    public virtual int IndexOf(string phrase, int startIndex, int count)  
    {  
        return someText.IndexOf(phrase, startIndex, count);
    }  
}

The class MyString provides three overloads for the IndexOf method, but two of them simply call the one with one more parameter. Notice that the same rule applies to class constructors; implement the most complete overload and call that one from the other overloads using the this() operator. Also notice that the parameters with the same name should appear in the same position in all overloads.

Important: If you also want to allow derived classes to override these methods, define the most complete overload as a non-private virtual method that is called by all overloads.

AV1553: Only use optional parameters to replace overloads

The only valid reason for using C# 4.0’s optional parameters is to replace the example from rule AV1551 with a single method like:

public virtual int IndexOf(string phrase, int startIndex = 0, int count = -1)
{
    int length = count == -1 ? someText.Length - startIndex : count;
    return someText.IndexOf(phrase, startIndex, length);
}

Since strings, collections and tasks should never be null according to rule AV1135, if you have an optional parameter of these types with default value null then you must use overloaded methods instead.

Strings, unlike other reference types, can have non-null default values. So an optional string parameter may be used to replace overloads with the condition of having a non-null default value.

Regardless of optional parameters’ types, following caveats always apply:

1) The default values of the optional parameters are stored at the caller side. As such, changing the default argument without recompiling the calling code will not apply the new default value. Unless your method is private or internal, this aspect should be carefully considered before choosing optional parameters over method overloads.

2) If optional parameters cause the method to follow and/or exit from alternative paths, overloaded methods are probably a better fit for your case.

AV1554: Do not use optional parameters in interface methods or their concrete implementations

When an interface method defines an optional parameter, its default value is discarded during overload resolution unless you call the concrete class through the interface reference.

When a concrete implementation of an interface method sets a default argument for a parameter, the default value is discarded during overload resolution if you call the concrete class through the interface reference.

See the series on optional argument corner cases by Eric Lippert (part one, two, three, four) for more details.

AV1555: Avoid using named arguments

C# 4.0’s named arguments have been introduced to make it easier to call COM components that are known for offering many optional parameters. If you need named arguments to improve the readability of the call to a method, that method is probably doing too much and should be refactored.

Exception: The only exception where named arguments improve readability is when calling a method of some code base you don’t control that has a bool parameter, like this:

object[] myAttributes = type.GetCustomAttributes(typeof(MyAttribute), inherit: false);

AV1561: Don’t declare signatures with more than 3 parameters

To keep constructors, methods, delegates and local functions small and focused, do not use more than three parameters. Do not use tuple parameters. Do not return tuples with more than two elements.

If you want to use more parameters, use a structure or class to pass multiple arguments, as explained in the Specification design pattern. In general, the fewer the parameters, the easier it is to understand the method. Additionally, unit testing a method with many parameters requires many scenarios to test.

Exception: A parameter that is a collection of tuples is allowed.

AV1562: Don’t use ref or out parameters

They make code less understandable and might cause people to introduce bugs. Instead, return compound objects or tuples.

Exception: Calling and declaring members that implement the TryParse pattern is allowed. For example:

bool success = int.TryParse(text, out int number);

AV1564: Avoid signatures that take a bool parameter

Consider the following method signature:

public Customer CreateCustomer(bool platinumLevel) 
{

}

On first sight this signature seems perfectly fine, but when calling this method you will lose this purpose completely:

Customer customer = CreateCustomer(true);

Often, a method taking such a bool is doing more than one thing and needs to be refactored into two or more methods. An alternative solution is to replace the bool with an enumeration.

AV1568: Don’t use parameters as temporary variables

Never use a parameter as a convenient variable for storing temporary state. Even though the type of your temporary variable may be the same, the name usually does not reflect the purpose of the temporary variable.

AV1570: Prefer is patterns over as operations

If you use ‘as’ to safely upcast an interface reference to a certain type, always verify that the operation does not return null. Failure to do so may cause a NullReferenceException at a later stage if the object did not implement that interface. Pattern matching syntax prevents this and improves readability. For example, instead of:

var remoteUser = user as RemoteUser;
if (remoteUser != null)
{
}

write:

if (user is RemoteUser remoteUser)
{
}

AV1575: Don’t comment out code

Never check in code that is commented out. Instead, use a work item tracking system to keep track of some work to be done. Nobody knows what to do when they encounter a block of commented-out code. Was it temporarily disabled for testing purposes? Was it copied as an example? Should I delete it?

AV1580: Write code that is easy to debug

Because debugger breakpoints cannot be set inside expressions, avoid overuse of nested method calls. For example, a line like:

string result = ConvertToXml(ApplyTransforms(ExecuteQuery(GetConfigurationSettings(source))));

requires extra steps to inspect intermediate method return values. On the other hard, were this expression broken into intermediate variables, setting a breakpoint on one of them would be sufficient.

Note This does not apply to chaining method calls, which is a common pattern in fluent APIs.

Naming Guidelines

AV1701: Use US English

All identifiers (such as types, type members, parameters and variables) should be named using words from the American English language.

DG1702: Use proper casing for language elements

Language element Casing Example
Namespace Pascal System.Drawing
Type parameter Pascal TView
Interface Pascal IBusinessService
Class, struct Pascal AppDomain
Enum Pascal ErrorLevel
Enum member Pascal FatalError
Resource key Pascal SaveButtonTooltipText
Private field (incl. const / static readonly) Camel listItem
Non-private field (incl. const / static readonly) Pascal MainPanel
Property Pascal BackColor
Event Pascal Click
Method Pascal ToString
Local function Pascal FormatText
Parameter Camel typeName
Tuple element names Pascal (string First, string Last) name = ("John", "Doe");
var name = (First: "John", Last: "Doe");
(string First, string Last) GetName() => ("John", "Doe");
Variables declared using tuple syntax Camel (string first, string last) = ("John", "Doe");
var (first, last) = ("John", "Doe");
    var (first, last) = ("John", "Doe");
Local variable Camel listOfValues

Note: in case of ambiguity, the rule higher in the table wins.

AV1704: Don’t include numbers in variables, parameters and type members

In most cases they are a lazy excuse for not defining a clear and intention-revealing name.

AV1705: Don’t prefix fields

For example, don’t use g_ or s_ to distinguish static from non-static fields. A method in which it is difficult to distinguish local variables from member fields is generally too big. Examples of incorrect identifier names are: _currentUser, mUserName, m_loginTime.

AV1706: Don’t use abbreviations

For example, use ButtonOnClick rather than BtnOnClick. Avoid single character variable names, such as i or q. Use index or query instead.

Exceptions: Use well-known acronyms and abbreviations that are widely accepted or well-known in your work domain. For instance, use acronym UI instead of UserInterface and abbreviation Id instead of Identity.

AV1707: Name members, parameters and variables according to their meaning and not their type

AV1708: Name types using nouns, noun phrases or adjective phrases

For example, the name IComponent uses a descriptive noun, ICustomAttributeProvider uses a noun phrase and IPersistable uses an adjective. Bad examples include SearchExamination (a page to search for examinations), Common (does not end with a noun, and does not explain its purpose) and SiteSecurity (although the name is technically okay, it does not say anything about its purpose).

Don’t include terms like Utility or Helper in classes. Classes with names like that are usually static classes and are introduced without considering object-oriented principles (see also AV1008).

AV1709: Name generic type parameters with descriptive names

AV1710: Don’t repeat the name of a class or enumeration in its members

class Employee
{
  // Wrong!
  static GetEmployee() {...}
  DeleteEmployee() {...}

  // Right
  static Get() {...}
  Delete() {...}

  // Also correct.
  AddNewJob() {...}
  RegisterForMeeting() {...}
}

.NET developers are already accustomed to the naming patterns the framework uses, so following this same pattern helps them find their way in your classes as well. For instance, if you define a class that behaves like a collection, provide members like Add, Remove and Count instead of AddItem, Delete or NumberOfItems.

AV1712: Avoid short names or names that can be mistaken for other names

Although technically correct, statements like the following can be confusing:

bool b001 = lo == l0 ? I1 == 11 : lOl != 101;

AV1715: Properly name properties

AV1720: Name methods and local functions using verbs or verb-object pairs

Name a method or local function using a verb like Show or a verb-object pair such as ShowDialog. A good name should give a hint on the what of a member, and if possible, the why.

Also, don’t include And in the name of a method or local function. That implies that it is doing more than one thing, which violates the Single Responsibility Principle explained in AV1115.

AV1725: Name namespaces using names, layers, verbs and features

For instance, the following namespaces are good examples of that guideline.

AvivaSolutions.Commerce.Web
NHibernate.Extensibility
Microsoft.ServiceModel.WebApi
Microsoft.VisualStudio.Debugging
FluentAssertion.Primitives
CaliburnMicro.Extensions

Note: Never allow namespaces to contain the name of a type, but a noun in its plural form (e.g. Collections) is usually OK.

AV1735: Use a verb or verb phrase to name an event

Name events with a verb or a verb phrase. For example: Click, Deleted, Closing, Minimizing, and Arriving. For example, the declaration of the Search event may look like this:

public event EventHandler<SearchArgs> Search;

AV1737: Use -ing and -ed to express pre-events and post-events

For example, a close event that is raised before a window is closed would be called Closing, and one that is raised after the window is closed would be called Closed. Don’t use Before or After prefixes or suffixes to indicate pre and post events.

Suppose you want to define events related to the deletion of an object. Avoid defining the Deleting and Deleted events as BeginDelete and EndDelete. Define those events as follows:

AV1738: Prefix an event handler with “On”

It is good practice to prefix the method that handles an event with “On”. For example, a method that handles its own Closing event should be named OnClosing. And a method that handles the Click event of its okButton field should be named OkButtonOnClick.

AV1739: Use an underscore for irrelevant lambda parameters

If you use a lambda expression (for instance, to subscribe to an event) and the actual parameters of the event are irrelevant, use the following convention to make that explicit:

button.Click += (_, __) => HandleClick();

Note If using C# 9 or higher, use a single underscore (discard) for all unused lambda parameters and variables.

AV1745: Group extension methods in a class suffixed with Extensions

If the name of an extension method conflicts with another member or extension method, you must prefix the call with the class name. Having them in a dedicated class with the Extensions suffix improves readability.

AV1755: Postfix asynchronous methods with Async or TaskAsync

The general convention for methods and local functions that return Task or Task<TResult> is to postfix them with Async. But if such a method already exists, use TaskAsync instead.

Performance Guidelines

AV1800: Consider using Any() to determine whether an IEnumerable<T> is empty

When a member or local function returns an IEnumerable<T> or other collection class that does not expose a Count property, use the Any() extension method rather than Count() to determine whether the collection contains items. If you do use Count(), you risk that iterating over the entire collection might have a significant impact (such as when it really is an IQueryable<T> to a persistent store).

Note: If you return an IEnumerable<T> to prevent changes from calling code as explained in AV1130, and you’re developing in .NET 4.5 or higher, consider the new read-only classes.

AV1820: Only use async for low-intensive long-running activities

The usage of async won’t automagically run something on a worker thread like Task.Run does. It just adds the necessary logic to allow releasing the current thread, and marshal the result back on that same thread if a long-running asynchronous operation has completed. In other words, use async only for I/O bound operations.

AV1825: Prefer Task.Run or Task.Factory.StartNew for CPU-intensive activities

If you do need to execute a CPU bound operation, use Task.Run to offload the work to a thread from the Thread Pool. For long-running operations use Task.Factory.StartNew with TaskCreationOptions.LongRunning parameter to create a new thread. Remember that you have to marshal the result back to your main thread manually.

AV1830: Beware of mixing up async/await with Task.Wait

await does not block the current thread but simply instructs the compiler to generate a state-machine. However, Task.Wait blocks the thread and may even cause deadlocks (see AV1835).

AV1835: Beware of async/await deadlocks in special environments (e.g. WPF)

Consider the following asynchronous method:

private async Task<string> GetDataAsync()
{
    var result = await MyWebService.GetDataAsync();
    return result.ToString();
}

Now when a button event handler is implemented like this:

public async void Button1_Click(object sender, RoutedEventArgs e)
{
    var data = GetDataAsync().Result;
    textBox1.Text = data;
}

You will likely end up with a deadlock. Why? Because the Result property getter will block until the async operation has completed, but since an async method could automatically marshal the result back to the original thread (depending on the current SynchronizationContext or TaskScheduler) and WPF uses a single-threaded synchronization context, they’ll be waiting on each other. A similar problem can also happen on UWP, WinForms, classical ASP.NET (not ASP.NET Core) or a Windows Store C#/XAML app. Read more about this here.

AV1840: Await ValueTask and ValueTask<T> directly and exactly once

The consumption of the newer and performance related ValueTask and ValueTask<T> types is more restrictive than consuming Task or Task<T>. Starting with .NET Core 2.1 the ValueTask<T> is not only able to wrap the result T or a Task<T>, with this version it is also possible to wrap a IValueTaskSource / IValueTaskSource<T> which gives the developer extra support for reuse and pooling. This enhanced support might lead to unwanted side-effects, as the ValueTask-returning developer might reuse the underlying object after it got awaited. The safest way to consume a ValueTask / ValueTask<T> is to directly await it once, or call .AsTask() to get a Task / Task<T> to overcome these limitations.

// OK / GOOD
int bytesRead = await stream.ReadAsync(buffer, cancellationToken);

// OK / GOOD
int bytesRead = await stream.ReadAsync(buffer, cancellationToken).ConfigureAwait(false);

// OK / GOOD - Get task if you want to overcome the limitations exposed by ValueTask / ValueTask<T>
Task<int> task = stream.ReadAsync(buffer, cancellationToken).AsTask();

Other usage patterns might still work (like saving the ValueTask / ValueTask<T> into a variable and awaiting later), but may lead to misuse eventually. Not awaiting a ValueTask / ValueTask<T> may also cause unwanted side-effects. Read more about ValueTask / ValueTask<T> and the correct usage here.

Framework Guidelines

AV2201: Use C# type aliases instead of the types from the System namespace

For instance, use object instead of Object, string instead of String, and int instead of Int32. These aliases have been introduced to make the primitive types first class citizens of the C# language, so use them accordingly. When referring to static members of those types, use int.Parse() instead of Int32.Parse().

Exception: For interop with other languages, it is custom to use the CLS-compliant name in type and member signatures, e.g. HexToInt32Converter, GetUInt16.

AV2202: Prefer language syntax over explicit calls to underlying implementations

Language syntax makes code more concise. The abstractions make later refactorings easier (and sometimes allow for extra optimizations).

Prefer:

(string, int) tuple = ("", 1);

rather than:

ValueTuple<string, int> tuple = new ValueTuple<string, int>("", 1);

Prefer:

DateTime? startDate;

rather than:

Nullable<DateTime> startDate;

Prefer:

if (startDate != null) ...

rather than:

if (startDate.HasValue) ...

Prefer:

if (startDate > DateTime.Now) ...

rather than:

if (startDate.HasValue && startDate.Value > DateTime.Now) ...

Prefer:

(DateTime startTime, TimeSpan duration) tuple1 = GetTimeRange();
(DateTime startTime, TimeSpan duration) tuple2 = GetTimeRange();

if (tuple1 == tuple2) ...

rather than:

if (tuple1.startTime == tuple2.startTime && tuple1.duration == tuple2.duration) ...

AV2207: Don’t hard-code strings that change based on the deployment

Examples include connection strings, server addresses, etc. Use Resources, the ConnectionStrings property of the ConfigurationManager class, or the Settings class generated by Visual Studio. Maintain the actual values into the app.config or web.config (and most definitely not in a custom configuration store).

AV2210: Build with the highest warning level

Configure the development environment to use the highest available warning level for the C# compiler, and enable the option Treat warnings as errors. This allows the compiler to enforce the highest possible code quality.

AV2220: Avoid LINQ query syntax for simple expressions

Rather than:

var query = from item in items where item.Length > 0 select item;

prefer the use of extension methods from the System.Linq namespace:

var query = items.Where(item => item.Length > 0);

The second example is a bit less convoluted.

AV2221: Use lambda expressions instead of anonymous methods

Lambda expressions provide a more elegant alternative for anonymous methods. So instead of:

Customer customer = Array.Find(customers, delegate(Customer customer)
{
    return customer.Name == "Tom";
});

use a lambda expression:

Customer customer = Array.Find(customers, customer => customer.Name == "Tom");

Or even better:

var customer = customers.FirstOrDefault(customer => customer.Name == "Tom");

AV2230: Only use the dynamic keyword when talking to a dynamic object

The dynamic keyword has been introduced for interop with languages where properties and methods can appear and disappear at runtime. Using it can introduce a serious performance bottleneck, because various compile-time checks (such as overload resolution) need to happen at runtime, again and again on each invocation. You’ll get better performance using cached reflection lookups, Activator.CreateInstance() or pre-compiled expressions (see here for examples and benchmark results).

While using dynamic may improve code readability, try to avoid it in library code (especially in hot code paths). However, keep things in perspective: we’re talking microseconds here, so perhaps you’ll gain more by optimizing your SQL statements first.

AV2235: Favor async/await over Task continuations

Using the new C# 5.0 keywords results in code that can still be read sequentially and also improves maintainability a lot, even if you need to chain multiple asynchronous operations. For example, rather than defining your method like this:

public Task<Data> GetDataAsync()
{
    return MyWebService.FetchDataAsync()
      .ContinueWith(t => new Data(t.Result));
}

define it like this:

public async Task<Data> GetDataAsync()
{
    string result = await MyWebService.FetchDataAsync();
    return new Data(result);
}

Tip: Even if you need to target .NET Framework 4.0 you can use the async and await keywords. Simply install the Async Targeting Pack.

Documentation Guidelines

AV2301: Write comments and documentation in US English

AV2305: Document all public, protected and internal types and members

Documenting your code allows Visual Studio, Visual Studio Code or Jetbrains Rider to pop-up the documentation when your class is used somewhere else. Furthermore, by properly documenting your classes, tools can generate professionally looking class documentation.

AV2306: Write XML documentation with other developers in mind

Write the documentation of your type with other developers in mind. Assume they will not have access to the source code and try to explain how to get the most out of the functionality of your type.

AV2307: Write MSDN-style documentation

Following the MSDN online help style and word choice helps developers find their way through your documentation more easily.

AV2310: Avoid inline comments

If you feel the need to explain a block of code using a comment, consider replacing that block with a method with a clear name.

AV2316: Only write comments to explain complex algorithms or decisions

Try to focus comments on the why and what of a code block and not the how. Avoid explaining the statements in words, but instead help the reader understand why you chose a certain solution or algorithm and what you are trying to achieve. If applicable, also mention that you chose an alternative solution because you ran into a problem with the obvious solution.

AV2318: Don’t use comments for tracking work to be done later

Annotating a block of code or some work to be done using a TODO or similar comment may seem a reasonable way of tracking work-to-be-done. But in reality, nobody really searches for comments like that. Use a work item tracking system to keep track of leftovers.

Layout Guidelines

DG2400: Use a common layout

var dto = new ConsumerDto
{
    Id = 123,
    Name = "Microsoft",
    PartnerShip = PartnerShip.Gold,
    ShoppingCart =
    {
        ["VisualStudio"] = 1
    }
};

methodThatTakesAnAction.Do(x =>
{ 
    // do something like this 
}

private string GetLongText =>
    "ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC";

var query = from product in products where product.Price > 10 select product;

or

var query =  
    from product in products  
    where product.Price > 10  
    select product;

DG2402: Order namespaces in alphabetic order

using AvivaSolutions.Business;
using AvivaSolutions.Standard;
using System;
using System.Collections.Generic;
using System.Xml;
using Telerik.WebControls;
using Telerik.Ajax;

Using static directives and using alias directives should be written below regular using directives. Always place these directives at the top of the file, before any namespace declarations (not inside them).

DG2406: Place members in a well-defined order

Maintaining a common order allows other team members to find their way in your code more easily. In general, a source file should be readable from top to bottom, as if reading a book, to prevent readers from having to browse up and down through the code file.

  1. Public Delegates, Enums
  2. Static fields and constants
  3. Events
  4. Fields
  5. Properties, Indexers
  6. Constructors and the finalizer
  7. All other members in calling order
  8. Nested Types

Declare local functions at the bottom of their containing method bodies (after all executable code).

AV2407: Do not use #region

Regions require extra work without increasing the quality or the readability of code. Instead they make code harder to view and refactor.

AV2410: Use expression-bodied members appropriately

Favor expression-bodied member syntax over regular member syntax only when:

About this document

In addition to the many links provided throughout this document, I’d like to recommend the following books, articles and sites for everyone interested in software quality:

What is this

This document attempts to provide guidelines (or coding standards if you like) for all versions of C# up to and including v10 that are both valuable and pragmatic. Of course, if you create such a document you should practice what you preach. So rest assured, these guidelines are representative to what we at Aviva Solutions do in our day-to-day work. Notice that not all guidelines have a clear rationale. Some of them are simply choices we made at Aviva Solutions. In the end, it doesn’t matter what choice you made, as long as you make one and apply it consistently.

These guidelines are based on: https://csharpcodingguidelines.com/ by Dennis Doomen of Aviva solutions. Most of the rules are kept as is, but some are updated.

What is different compared to Aviva / CSharpGuidelines

Some rules have been changed to match personal preferences. I renamed the rule code to be prefixed with DG instead of AV to avoid confusion.

Why would you use this document

Although some might see coding guidelines as undesired overhead or something that limits creativity, this approach has already proven its value for many years. This is because not every developer:

Basic principles

There are many unexpected things I run into during my work as a consultant, each deserving at least one guideline. Unfortunately, I still need to keep this document within a reasonable size. But unlike what some junior developers believe, that doesn’t mean that something is okay just because it is not mentioned in this document.

In general, if I have a discussion with a colleague about a smell that this document does not cover, I’ll refer back to a set of basic principles that apply to all situations, regardless of context. These include:

Regardless of the elegance of someone’s solution, if it’s too complex for the ordinary developer, exposes unusual behavior, or tries to solve many possible future issues, it is very likely the wrong solution and needs redesign. The worst response a developer can give you to these principles is: “But it works?”.

How do you get started

Why did Dennis Doomen create it

The idea started in 2002 when Vic Hartog (Philips Medical Systems) and I were assigned the task of writing up a coding standard for C# 1.0. Since then, I’ve regularly added, removed and changed rules based on experiences, feedback from the community and new tooling support offered by a continuous stream of new developments in the .NET ecosystem. Special thanks go to Bart Koelman for being a very active contributor over all those years.

Additionally, after reading Robert C. Martin’s book Clean Code: A Handbook of Agile Software Craftsmanship, I became a big fan of his ideas and decided to include some of his smells and heuristics as guidelines. Notice though, that this document is in no way a replacement for his book. I sincerely recommend that you read his book to gain a solid understanding of the rationale behind his recommendations.

I’ve also decided to include some design guidelines in addition to simple coding guidelines. They are too important to ignore and have a big influence in reaching high quality code.

Is this a coding standard

The document does not state that projects must comply with these guidelines, neither does it say which guidelines are more important than others. However, we encourage projects to decide themselves which guidelines are important, what deviations a project will use, who is the consultant in case doubts arise, and what kind of layout must be used for source code. Obviously, you should make these decisions before starting the real coding work.

Feedback and disclaimer

This document has been compiled using many contributions from community members, blog posts, on-line discussions and two decades of developing in C#. If you have questions, comments or suggestions, just let me know by sending me an email at dennis.doomen@avivasolutions.nl, creating an issue or Pull Request on GitHub, ping me at http://twitter.com/ddoomen or join the Gitter discussions. I will try to revise and republish this document with new insights, experiences and remarks on a regular basis.

Notice though that it merely reflects my view on proper C# code so Aviva Solutions will not be liable for any direct or indirect damages caused by applying the guidelines of this document. This document is published under a Creative Commons license, specifically the Creative Commons Attribution-ShareAlike 4.0 license.

Can I create my own version

Absolutely. The corresponding license allows you to fork, adapt and distribute that modified version within your organization as long as you refer back to the original version here. It’s not required, but you would make me a very happy man if you credit me as the original author. And if you have any great ideas, recommendations or corrections, either submit an issue, or even better, fork the repository and provide me with a pull request.