C# gotcha

Consider you have the following code:

SomeClass reference = null;
Console.WriteLine(reference.Echo("echo"));

Your first thought might be “NullReferenceException”. Ha, think again! What if you have an class defining an extension method like this:

public static class SomeClassExt 
{
   public static string Echo(this SomeClass x, string str)
   {
       return str;
   }
}

You have been warned :)