Please explain extension methods to me

I just looked at this posting: What is the best or most interesting use of Extension Methods 开发者_如何转开发you’ve seen?

I've never heard of extension methods. Do they apply to every language?

What is the point of them? In that particular posting I did not understand the example.

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No, they do not apply to every language. They are a language-specific feature, offered by C# and Visual Basic (other languages may have adopted them since, I don't know).

The point of them is to provide a convenient syntax for calling utility methods on a class or interface. In C# 2, a utility method would be called through a static class, passing the argument in the normal way:

IEnumerable<string> someStrings;
int count = EnumerableHelpers.Count(someStrings);

With extension methods, this can be written more conveniently using something that looks like normal member method syntax:

int count = someStrings.Count();

even though Count() is not a member of the IEnumerable<string> interface. So extension methods let you appear to add members to classes or interfaces that you don't control.

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They are available to C# and VB. They allow you to simulate the addition of methods to classes that are not under your control.

You could, for instance, you could add a WordCount method to string.

So instead of

MyStringUtils.WordCount( "some string" )

You could just write "some string".WordCount().

public static class MyExtensions
{
    public static int WordCount(this String str)
    {
        return MyStringUtils.WordCount( str );
    }
} 

This is strictly "syntactic sugar" and aids in readability. You're essentially creating a static method that will be shown in the IntelliSense for the target type.

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do they apply to every language?

No, but some languages have the same feature, though not necessarily under the same name. Those I know about are Objective-C and Groovy, where the feature is called "Categories".