Can "this" be null in C# virtual methods? What happens with the rest of instance methods?

Question

I was curious if there is a way for this to be null in a virtual method in C#. I assume it is not possible. I saw that in existing code, during a code review and I would like to be 100% sure to comment for its removal, but I would like some confirmation and some more context from the community. Is it the case that this != null in any non-static / instance method? Otherwise it would have been a null pointer exception right? I was thinking of extension methods and such or any C# feature that I could possibly be not familiar with coming from years of Java.

Answer 1

It's not standard C# but, further to the answers from Lasse and Jon, with a bit of IL-fiddling you can make a non-virtual call (to either virtual or non-virtual methods) passing a null this:

using System;
using System.Reflection.Emit;

class Test
{
    static void Main()
    {
        CallWithNullThis("Foo");
        CallWithNullThis("Bar");
    }

    static void CallWithNullThis(string methodName)
    {
        var mi = typeof(Test).GetMethod(methodName);

        // make Test the owner type to avoid VerificationException
        var dm = new DynamicMethod("$", typeof(void), Type.EmptyTypes, typeof(Test));
        var il = dm.GetILGenerator();
        il.Emit(OpCodes.Ldnull);
        il.Emit(OpCodes.Call, mi);
        il.Emit(OpCodes.Ret);

        var action = (Action)dm.CreateDelegate(typeof(Action));
        action();
    }

    public void Foo()
    {
        Console.WriteLine(this == null ? "Weird" : "Normal");
    }

    public virtual void Bar()
    {
        Console.WriteLine(this == null ? "Weird" : "Normal");
    }
}