What is the best or most interesting use of Extension Methods you've seen? [closed]

Answer 1

This one creates array with single element added at the very beginning:

public static T[] Prepend(this T[] array, T item)
{
    T[] result = new T[array.Length + 1];
    result[0] = item;
    Array.Copy(array, 0, result, 1, array.Length);
    return result;
}

string[] some = new string[] { "foo", "bar" };
...
some = some.Prepend("baz"); 

And this one helps me when I need to convert some expression to it's square:

public static double Sq(this double arg)
{
    return arg * arg;
}

(x - x0).Sq() + (y - y0).Sq() + (z - z0).Sq()