Why can extension method not overwrite original object?

Question

It seems that an extension method in C# cannot overwrite the original object. Why is that? Example:

using System;

namespace ExtensionTest
{
    public class MyTest {
        public string MyName { get; set; } 
    }


    class Program
   {
        static void Main(string[] args)
        {
            var myTest = new MyTest() { MyName = "Arne" };
            Console.WriteLine("My name is {0}", myTest.MyName);
            // Will write "My name is Arne"
            myTest.AlterMyTest();
            Console.WriteLine("My name is {0}", myTest.MyName);
            // Will write "My name is Bertil"
            myTest.OverwriteMyTest();
            Console.WriteLine("My name is {0}", myTest.MyName);
            // Will write "My name is Bertil" (why?)
        }
    }

    public static class ExtensionClass{
        public static void AlterMyTest(this MyTest myTest)
        {
            myTest.MyName = "Bertil";
        }

         public static void OverwriteMyTest(this MyTest myTest)
         {
             myTest = new MyTest() { MyName = "Carl" };
         }
    }
}

Answer 1

Because as usual, reference of the class is copied while passing to the method, and you are assigning new object to the new reference.

For not-extension methods, you can pass reference by ref/out keywords

public static void Func(out MyClass b)
{
   b = new MyClass();
}

...
MyClass b;
Func(out b);
Assert.IsNotNull(b);

but C# compiler doesn't allow to use ref with this(the reason is in David Arno's comment). You are free to remove this keyword, and call static method instead of extension.