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C# Extension Classes

Context

Developer wants to implement additional methods for a class as if those methods are the member methods of the class. For example, instead of implementing a function ToInt() used as follows:

string x = '32'
var y = ToInt(x)

The developer wants the said function to be invoked as follows:

string x = '32'
var y = x.ToInt()

The developer does not have the permission to modify the class and cannot directly add the addition methods to the class.

Problem

How to add such "additional methods" to a class that the developer does not have permission to modify the source code?

Solution

Assume name of the class of interest is ClassX and the signature of the method to be added is int MethodY(ClassX x), implement the following class, which is referred to as the "extension class" of ClassX:

namespace Some.Namespace.For.ClassXExtension

public static class ClassXExtension
{
    public static int MethodY(this ClassX x)
    {
        // Implementation
    }
}

Now, in a different part of the project, the developer will be able to invoke MethodY as if it's a member method of ClassX (as long as that part of the project recognizes the namespace Some.Namespace.For.ClassXExtension).

Note that:

  • The extension class name does not matter. But the convention is to name it as <Class_Of_Interest>Extension

Reference