In object-oriented computer programming, an extension method is a method added to an object after the original object was compiled. The modified object is often a class, a prototype or a type. Extension methods are features of some object-oriented programming languages. There is no syntactic difference between calling an extension method and calling a method declared in the type definition.^[1]

Not all languages implement extension methods in an equally safe manner, however. For instance, languages such as C#, Java (via Manifold, Lombok, or Fluent), and Kotlin don't alter the extended class in any way, because doing so may break class hierarchies and interfere with virtual method dispatching. This is why these languages strictly implement extension methods statically and use static dispatching to invoke them.

Support in programming languages

Extension methods are features of numerous languages including C#, Java via Manifold or Lombok or Fluent, Gosu, JavaScript, Oxygene, Ruby, Smalltalk, Kotlin, Dart, Visual Basic.NET and Xojo. In dynamic languages like Python, the concept of an extension method is unnecessary because non-builtin classes can be extended without any special syntax (an approach known as "monkey-patching", employed in libraries such as gevent).

In VB.NET and Oxygene, they are recognized by the presence of the "extension" keyword or attribute. In Xojo the "Extends" keyword is used with global methods.

In C# they're implemented as static methods in static classes, with the first argument being of extended class and preceded by "this" keyword.

In Java you add extension methods via Manifold, a jar file you add to your project's classpath. Similar to C# a Java extension method is declared static in an @Extension class where the first argument has the same type as the extended class and is annotated with @This. Alternatively, the Fluent plugin allows you to call any static method as an extension method without using annotations, as long as the method signature matches.

In Smalltalk, any code can add a method to any class at any time, by sending a method creation message (such as methodsFor:) to the class the user wants to extend. The Smalltalk method category is conventionally named after the package that provides the extension, surrounded by asterisks. For example, when Etoys application code extends classes in the core library, the added methods are put in the *etoys* category.

In Ruby, like Smalltalk, there is no special language feature for extension, as Ruby allows classes to be re-opened at any time with the class keyword, in this case, to add new methods. The Ruby community often describes an extension method as a kind of monkey patch. There is also a newer feature for adding safe/local extensions to the objects, called Refinements, but it is known to be less used.

In Swift, the extension keyword marks a class-like construct that allows the addition of methods, constructors, and fields to an existing class, including the ability to implement a new interface/protocol to the existing class.^[2]

Extension methods as enabling feature

Next to extension methods allowing code written by others to be extended as described below, extension methods enable patterns that are useful in their own right as well. The predominant reason why extension methods were introduced was Language Integrated Query (LINQ). Compiler support for extension methods allows deep integration of LINQ with old code just the same as with new code, as well as support for query syntax which for the moment is unique to the primary Microsoft .NET languages.

Console.WriteLine(new[] { Math.PI, Math.E }.Where(d => d > 3).Select(d => Math.Sin(d / 2)).Sum());
// Output:
// 1

namespace MyCoolLogger;

public interface ILogger
{
    void Write(string text);
}

public static class LoggerExtensions
{
    public static void Write(this ILogger logger, string format, params object[] args)
    {
        if (logger != null)
            logger.Write(string.Format(format, args));
    }
}

public class TodoItemContext : DbContext 
{
    public DbSet<TodoItem> TodoItems { get; set; }
    public DbSet<TodoList> TodoLists { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) 
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<TodoList>()
                    .HasKey(e => e.TodoListId)
                    .HasEntitySetName("Lists")
                    .HasMany(e => e.Todos)
                    .WithRequired(e => e.TodoList);
    }
}

public class BankAccount : IEnumerable<decimal> 
{
    private List<Tuple<DateTime, decimal>> credits; // assumed all negative
    private List<Tuple<DateTime, decimal>> debits; // assumed all positive

    public IEnumerator<decimal> GetEnumerator() 
    {
        var query = from dc in debits.Union(credits) 
                    orderby dc.Item1 /* Date */ 
                    select dc.Item2; /* Amount */
    
        foreach (var amount in query)
            yield return amount;
    }
}
// given an instance of BankAccount called ba and a using System.Linq on top of the current file,
// one could now write ba.Sum() to get the account balance, ba.Reverse() to see most recent transactions first,
// ba.Average() to get the average amount per transaction, etcetera - without ever writing down an arithmetic operator

The problem

In programming, situations arise where it is necessary to add functionality to an existing class—for instance by adding a new method. Normally the programmer would modify the existing class's source code, but this forces the programmer to recompile all binaries with these new changes and requires that the programmer be able to modify the class, which is not always possible, for example when using classes from a third-party assembly. This is typically worked around in one of three ways, all of which are somewhat limited and unintuitive :

1. Inherit the class and then implement the functionality in an instance method in the derived class.
2. Implement the functionality in a static method added to a helper class.
3. Use aggregation instead of inheritance.

Current C# solutions

The first option is in principle easier, but it is unfortunately limited by the fact that many classes restrict inheritance of certain members or forbid it completely. This includes sealed class and the different primitive data types in C# such as int, float and string. The second option, on the other hand, does not share these restrictions, but it may be less intuitive as it requires a reference to a separate class instead of using the methods of the class in question directly.

As an example, consider a need of extending the string class with a new reverse method whose return value is a string with the characters in reversed order. Because the string class is a sealed type, the method would typically be added to a new utility class in a manner similar to the following:

string x = "some string value";
string y = Utility.Reverse(x);

This may, however, become increasingly difficult to navigate as the library of utility methods and classes increases, particularly for newcomers. The location is also less intuitive because, unlike most string methods, it would not be a member of the string class, but in a completely different class altogether. A better syntax would therefore be the following:

string x = "some string value";
string y = x.Reverse();

Current VB.NET solutions

In most ways, the VB.NET solution is similar to the C# solution above. However VB.NET has a unique advantage in that it allows members to be passed in to the extension by reference (C# only allows by value). Allowing for the following;

Dim x As String = "some string value"
x.Reverse()

Because Visual Basic allows the source object to be passed in by reference it is possible to make changes to the source object directly, without need to create another variable. It is also more intuitive as it works in a consistent fashion to existing methods of classes.

Extension methods

The new language feature of extension methods in C# 3.0, however, makes the latter code possible. This approach requires a static class and a static method, as follows.

public static class Utility
{
    public static string Reverse(this string input)
    {
        char[] chars = input.ToCharArray();
        Array.Reverse(chars);
        return new String(chars);
    }
}

In the definition, the modifier 'this' before the first argument specifies that it's an extension method (in this case to the type 'string'). In a call, the first argument is not 'passed in' because it is already known as the 'calling' object (the object before the dot).

The major difference between calling extension methods and calling static helper methods is that static methods are called in prefix notation, whereas extension methods are called in infix notation. The latter leads to more readable code when the result of one operation is used for another operation.

With static methods

HelperClass.Operation2(HelperClass.Operation1(x, arg1), arg2)

With extension methods

x.Operation1(arg1).Operation2(arg2)

Naming conflicts in extension methods and instance methods

In C# 3.0, both an instance method and an extension method with the same signature can exist for a class. In such a scenario, the instance method is preferred over the extension method. Neither the compiler nor the Microsoft Visual Studio IDE warns about the naming conflict. Consider this C# class, where the GetAlphabet() method is invoked on an instance of this class:

class AlphabetMaker 
{
    public void GetAlphabet()       
    {                               //When this method is implemented,
        Console.WriteLine("abc");   //it will shadow the implementation
    }                               //in the ExtensionMethods class.
}

static class ExtensionMethods
{
    public static void GetAlphabet(this AlphabetMaker am)   
    {                               //This will only be called                       
        Console.WriteLine("ABC");   //if there is no instance
    }                               //method with the same signature.   
}

Result of invoking GetAlphabet() on an instance of AlphabetMaker if only the extension method exists:

ABC

Result if both the instance method and the extension method exist:

abc

See also

• UFCS, a way to use free functions as extension methods provided in the D programming language
• type classes
• Anonymous types
• Lambda expressions
• Expression trees
• Runtime alteration
• Duck typing

References

External links

• Open source collection of C# extension methods libraries. Now archived at Codeplex
• Extension method in C#
• Extension methods
• C# Extension Methods. A collection.
• extensionmethod.net Large database with C#, Visual Basic, F# and Javascript extension methods
• Explanation and code example
• Defining your own functions in jQuery
• Uniform function call syntax
• Extension methods in C#
• Extension Methods in Java with Manifold
• Extension Methods in Java with Lombok
• Extension Methods in Java with Fluent
• Extension functions in Kotlin