Extension methods series: the basics

One the new thing in C# is extension methods. Extension methods are static method that works pretty much like an helper function but instead of passing the instance to act on as an argument the instance is a prefix to the method. It looks exactly like if the method is part of the type itself.

Of course because C# is a statically typed language we are not really adding new method to a type. Here is the trick. The following example is a classic case of helper function:

if (String.IsNullOrEmpty(instance))
{
	// ...
}

The method “IsNullOrEmpty” is static a member of type “String”. To use it we must call it by its type and then pass it an instance of that type. Here is an easy way transform this helper function into an extension method.

public static class StringExtensions
{
	public static bool IsNullOrEmpty(this string instance)
	{
	    return String.IsNullOrEmpty(instance);
	}	
}

One of the requirement to make extension methods is the class must be static. Notice the “this” keyword used on the first argument of the method. This tell the compiler which type this method is extending and what type it should be.

Now if we use this code it will look like this:

if (instance.IsNullOrEmpty())
{
	// ...
}

At compile time the condition will be replaced by “StringExtension.IsNullOrEmpty(instance)”. So extension method is just a compiler trick to facilitate the uses of helper function. Beside the fact that it is easier to use it is also more readable. That is the starting point to fluent interfaces.