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// Extend IBasicMath this method and this

// implementation.

public static int Subtract(this IBasicMath itf,

int x, int y)

{

return x - y;

}

}

At this point, you might assume you could create a variable of type IBasicMath and directly

invoke Subtract(). Again, if this were possible (which it is not), this would destroy the nature of

.NET interface types. In reality, what we have actually said here is “Any class in my project imple-

menting IBasicMath now has a Subtract() method, implemented in this manner.” As before, all the

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basic rules apply, therefore the namespace defining MyCalc must have access to the namespace

defining MathExtensions. Consider the following Main() method:

static void Main(string[] args)

{

Console.WriteLine("***** Extending an interface *****\n");