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How to implement and call a custom extension method (C# Programming Guide)

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This article shows how to implement your own extension methods for any .NET type. Client code can use your extension methods. Client projects must reference the assembly that contains them. Client projects must add a using directive that specifies the namespace in which the extension methods are defined.

To define and call the extension method:

  1. Define a static class to contain the extension method. The class can't be nested inside another type and must be visible to client code. For more information about accessibility rules, see Access Modifiers.
  2. Implement the extension method as a static method with at least the same visibility as the containing class.
  3. The first parameter of the method specifies the type that the method operates on; it must be preceded with the this modifier.
  4. In the calling code, add a using directive to specify the namespace that contains the extension method class.
  5. Call the methods as instance methods on the type.

Note

The first parameter is not specified by calling code because it represents the type on which the operator is being applied, and the compiler already knows the type of your object. You only have to provide arguments for parameters 2 through n.

The following example implements an extension method named WordCount in the CustomExtensions.StringExtension class. The method operates on the String class, which is specified as the first method parameter. The CustomExtensions namespace is imported into the application namespace, and the method is called inside the Main method.

using CustomExtensions;

string s = "The quick brown fox jumped over the lazy dog.";
// Call the method as if it were an
// instance method on the type. Note that the first
// parameter is not specified by the calling code.
int i = s.WordCount();
System.Console.WriteLine("Word count of s is {0}", i);


namespace CustomExtensions
{
    // Extension methods must be defined in a static class.
    public static class StringExtension
    {
        // This is the extension method.
        // The first parameter takes the "this" modifier
        // and specifies the type for which the method is defined.
        public static int WordCount(this string str)
        {
            return str.Split(new char[] {' ', '.','?'}, StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }
}

Overload resolution prefers instance or static method defined by the type itself to extension methods. Extension methods can't access any private data in the extended class.

See also