Extension Methods (C# Programming Guide)

Article
03/15/2024

Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are static methods, but they're called as if they were instance methods on the extended type. For client code written in C#, F# and Visual Basic, there's no apparent difference between calling an extension method and the methods defined in a type.

The most common extension methods are the LINQ standard query operators that add query functionality to the existing System.Collections.IEnumerable and System.Collections.Generic.IEnumerable<T> types. To use the standard query operators, first bring them into scope with a using System.Linq directive. Then any type that implements IEnumerable<T> appears to have instance methods such as GroupBy, OrderBy, Average, and so on. You can see these additional methods in IntelliSense statement completion when you type "dot" after an instance of an IEnumerable<T> type such as List<T> or Array.

OrderBy Example

The following example shows how to call the standard query operator OrderBy method on an array of integers. The expression in parentheses is a lambda expression. Many standard query operators take lambda expressions as parameters, but this isn't a requirement for extension methods. For more information, see Lambda Expressions.

class ExtensionMethods2
{

    static void Main()
    {
        int[] ints = [10, 45, 15, 39, 21, 26];
        var result = ints.OrderBy(g => g);
        foreach (var i in result)
        {
            System.Console.Write(i + " ");
        }
    }
}
//Output: 10 15 21 26 39 45

Extension methods are defined as static methods but are called by using instance method syntax. Their first parameter specifies which type the method operates on. The parameter follows the this modifier. Extension methods are only in scope when you explicitly import the namespace into your source code with a using directive.

The following example shows an extension method defined for the System.String class. It's defined inside a non-nested, non-generic static class:

namespace ExtensionMethods
{
    public static class MyExtensions
    {
        public static int WordCount(this string str)
        {
            return str.Split(new char[] { ' ', '.', '?' },
                             StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }
}

The WordCount extension method can be brought into scope with this using directive:

using ExtensionMethods;

And it can be called from an application by using this syntax:

string s = "Hello Extension Methods";
int i = s.WordCount();

You invoke the extension method in your code with instance method syntax. The intermediate language (IL) generated by the compiler translates your code into a call on the static method. The principle of encapsulation isn't really being violated. Extension methods can't access private variables in the type they're extending.

Both the MyExtensions class and the WordCount method are static, and it can be accessed like all other static members. The WordCount method can be invoked like other static methods as follows:

string s = "Hello Extension Methods";
int i = MyExtensions.WordCount(s);

The preceding C# code:

Declares and assigns a new string named s with a value of "Hello Extension Methods".
Calls MyExtensions.WordCount given argument s.

For more information, see How to implement and call a custom extension method.

In general, you'll probably be calling extension methods far more often than implementing your own. Because extension methods are called by using instance method syntax, no special knowledge is required to use them from client code. To enable extension methods for a particular type, just add a using directive for the namespace in which the methods are defined. For example, to use the standard query operators, add this using directive to your code:

using System.Linq;

(You may also have to add a reference to System.Core.dll.) You'll notice that the standard query operators now appear in IntelliSense as additional methods available for most IEnumerable<T> types.

Binding Extension Methods at Compile Time

You can use extension methods to extend a class or interface, but not to override them. An extension method with the same name and signature as an interface or class method will never be called. At compile time, extension methods always have lower priority than instance methods defined in the type itself. In other words, if a type has a method named Process(int i), and you have an extension method with the same signature, the compiler will always bind to the instance method. When the compiler encounters a method invocation, it first looks for a match in the type's instance methods. If no match is found, it searches for any extension methods that are defined for the type, and bind to the first extension method that it finds.

Example

The following example demonstrates the rules that the C# compiler follows in determining whether to bind a method call to an instance method on the type, or to an extension method. The static class Extensions contains extension methods defined for any type that implements IMyInterface. Classes A, B, and C all implement the interface.

The MethodB extension method is never called because its name and signature exactly match methods already implemented by the classes.

When the compiler can't find an instance method with a matching signature, it will bind to a matching extension method if one exists.

// Define an interface named IMyInterface.
namespace DefineIMyInterface
{
    public interface IMyInterface
    {
        // Any class that implements IMyInterface must define a method
        // that matches the following signature.
        void MethodB();
    }
}

// Define extension methods for IMyInterface.
namespace Extensions
{
    using System;
    using DefineIMyInterface;

    // The following extension methods can be accessed by instances of any
    // class that implements IMyInterface.
    public static class Extension
    {
        public static void MethodA(this IMyInterface myInterface, int i)
        {
            Console.WriteLine
                ("Extension.MethodA(this IMyInterface myInterface, int i)");
        }

        public static void MethodA(this IMyInterface myInterface, string s)
        {
            Console.WriteLine
                ("Extension.MethodA(this IMyInterface myInterface, string s)");
        }

        // This method is never called in ExtensionMethodsDemo1, because each
        // of the three classes A, B, and C implements a method named MethodB
        // that has a matching signature.
        public static void MethodB(this IMyInterface myInterface)
        {
            Console.WriteLine
                ("Extension.MethodB(this IMyInterface myInterface)");
        }
    }
}

// Define three classes that implement IMyInterface, and then use them to test
// the extension methods.
namespace ExtensionMethodsDemo1
{
    using System;
    using Extensions;
    using DefineIMyInterface;

    class A : IMyInterface
    {
        public void MethodB() { Console.WriteLine("A.MethodB()"); }
    }

    class B : IMyInterface
    {
        public void MethodB() { Console.WriteLine("B.MethodB()"); }
        public void MethodA(int i) { Console.WriteLine("B.MethodA(int i)"); }
    }

    class C : IMyInterface
    {
        public void MethodB() { Console.WriteLine("C.MethodB()"); }
        public void MethodA(object obj)
        {
            Console.WriteLine("C.MethodA(object obj)");
        }
    }

    class ExtMethodDemo
    {
        static void Main(string[] args)
        {
            // Declare an instance of class A, class B, and class C.
            A a = new A();
            B b = new B();
            C c = new C();

            // For a, b, and c, call the following methods:
            //      -- MethodA with an int argument
            //      -- MethodA with a string argument
            //      -- MethodB with no argument.

            // A contains no MethodA, so each call to MethodA resolves to
            // the extension method that has a matching signature.
            a.MethodA(1);           // Extension.MethodA(IMyInterface, int)
            a.MethodA("hello");     // Extension.MethodA(IMyInterface, string)

            // A has a method that matches the signature of the following call
            // to MethodB.
            a.MethodB();            // A.MethodB()

            // B has methods that match the signatures of the following
            // method calls.
            b.MethodA(1);           // B.MethodA(int)
            b.MethodB();            // B.MethodB()

            // B has no matching method for the following call, but
            // class Extension does.
            b.MethodA("hello");     // Extension.MethodA(IMyInterface, string)

            // C contains an instance method that matches each of the following
            // method calls.
            c.MethodA(1);           // C.MethodA(object)
            c.MethodA("hello");     // C.MethodA(object)
            c.MethodB();            // C.MethodB()
        }
    }
}
/* Output:
    Extension.MethodA(this IMyInterface myInterface, int i)
    Extension.MethodA(this IMyInterface myInterface, string s)
    A.MethodB()
    B.MethodA(int i)
    B.MethodB()
    Extension.MethodA(this IMyInterface myInterface, string s)
    C.MethodA(object obj)
    C.MethodA(object obj)
    C.MethodB()
 */

Common Usage Patterns

Collection Functionality

In the past, it was common to create "Collection Classes" that implemented the System.Collections.Generic.IEnumerable<T> interface for a given type and contained functionality that acted on collections of that type. While there's nothing wrong with creating this type of collection object, the same functionality can be achieved by using an extension on the System.Collections.Generic.IEnumerable<T>. Extensions have the advantage of allowing the functionality to be called from any collection such as an System.Array or System.Collections.Generic.List<T> that implements System.Collections.Generic.IEnumerable<T> on that type. An example of this using an Array of Int32 can be found earlier in this article.

Layer-Specific Functionality

When using an Onion Architecture or other layered application design, it's common to have a set of Domain Entities or Data Transfer Objects that can be used to communicate across application boundaries. These objects generally contain no functionality, or only minimal functionality that applies to all layers of the application. Extension methods can be used to add functionality that is specific to each application layer without loading the object down with methods not needed or wanted in other layers.

public class DomainEntity
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

static class DomainEntityExtensions
{
    static string FullName(this DomainEntity value)
        => $"{value.FirstName} {value.LastName}";
}

Extending Predefined Types

Rather than creating new objects when reusable functionality needs to be created, we can often extend an existing type, such as a .NET or CLR type. As an example, if we don't use extension methods, we might create an Engine or Query class to do the work of executing a query on a SQL Server that may be called from multiple places in our code. However we can instead extend the System.Data.SqlClient.SqlConnection class using extension methods to perform that query from anywhere we have a connection to a SQL Server. Other examples might be to add common functionality to the System.String class, extend the data processing capabilities of the System.IO.Stream object, and System.Exception objects for specific error handling functionality. These types of use-cases are limited only by your imagination and good sense.

Extending predefined types can be difficult with struct types because they're passed by value to methods. That means any changes to the struct are made to a copy of the struct. Those changes aren't visible once the extension method exits. You can add the ref modifier to the first argument making it a ref extension method. The ref keyword can appear before or after the this keyword without any semantic differences. Adding the ref modifier indicates that the first argument is passed by reference. This enables you to write extension methods that change the state of the struct being extended (note that private members aren't accessible). Only value types or generic types constrained to struct (see struct constraint for more information) are allowed as the first parameter of a ref extension method. The following example shows how to use a ref extension method to directly modify a built-in type without the need to reassign the result or pass it through a function with the ref keyword:

public static class IntExtensions
{
    public static void Increment(this int number)
        => number++;

    // Take note of the extra ref keyword here
    public static void RefIncrement(this ref int number)
        => number++;
}

public static class IntProgram
{
    public static void Test()
    {
        int x = 1;

        // Takes x by value leading to the extension method
        // Increment modifying its own copy, leaving x unchanged
        x.Increment();
        Console.WriteLine($"x is now {x}"); // x is now 1

        // Takes x by reference leading to the extension method
        // RefIncrement changing the value of x directly
        x.RefIncrement();
        Console.WriteLine($"x is now {x}"); // x is now 2
    }
}

This next example demonstrates ref extension methods for user-defined struct types:

public struct Account
{
    public uint id;
    public float balance;

    private int secret;
}

public static class AccountExtensions
{
    // ref keyword can also appear before the this keyword
    public static void Deposit(ref this Account account, float amount)
    {
        account.balance += amount;

        // The following line results in an error as an extension
        // method is not allowed to access private members
        // account.secret = 1; // CS0122
    }
}

public static class AccountProgram
{
    public static void Test()
    {
        Account account = new()
        {
            id = 1,
            balance = 100f
        };

        Console.WriteLine($"I have ${account.balance}"); // I have $100

        account.Deposit(50f);
        Console.WriteLine($"I have ${account.balance}"); // I have $150
    }
}

General Guidelines

While it's still considered preferable to add functionality by modifying an object's code or deriving a new type whenever it's reasonable and possible to do so, extension methods have become a crucial option for creating reusable functionality throughout the .NET ecosystem. For those occasions when the original source isn't under your control, when a derived object is inappropriate or impossible, or when the functionality shouldn't be exposed beyond its applicable scope, Extension methods are an excellent choice.

For more information on derived types, see Inheritance.

When using an extension method to extend a type whose source code you aren't in control of, you run the risk that a change in the implementation of the type will cause your extension method to break.

If you do implement extension methods for a given type, remember the following points:

An extension method is not called if it has the same signature as a method defined in the type.
Extension methods are brought into scope at the namespace level. For example, if you have multiple static classes that contain extension methods in a single namespace named Extensions, they'll all be brought into scope by the using Extensions; directive.

For a class library that you implemented, you shouldn't use extension methods to avoid incrementing the version number of an assembly. If you want to add significant functionality to a library for which you own the source code, follow the .NET guidelines for assembly versioning. For more information, see Assembly Versioning.

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