The difference between member methods and extension methods

This thing jumped at me from behind. The difference between member methods and extension methods is that an extension method is not a member of the class. It is a static method, called with a syntax that makes it look like a member of a class when it is not. This means that calling an extension method when the member is null, will not cause NullReferenceException, and that is why you need to check arguments for null on all your extension methods. Proven by following unit tests

// Written with MsTest as unit test framework
namespace LiteMedia
{
    using System;
    using System.Linq;
    using Microsoft.VisualStudio.TestTools.UnitTesting;

[TestClass]
public class VerifyClaim
{
    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void FirstOrDefaultThrowsArgumentNullException()
    {
        /* Setup */
        string[] names = null;

        /* Test */
        names.FirstOrDefault();
    }

    [TestMethod]
    [ExpectedException(typeof(NullReferenceException))]
    public void UsingClassMembersOnNullReferenceWillThrowNullReferenceException()
    {
        /* Setup */
        string[] names = null;

        /* Test */
        string fail = names.ToString();
    }
}

}