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

It's not possible to do this in normal C# (i.e. calling a method or property in a normal way), regardless of whether the method is virtual or not.

For non-virtual methods, you can create a delegate from an open instance method, effectively treating the instance method as a static method with a first parameter with the target type. You can invoke that delegate with a null argument, and observe this == null in the method.

For a virtual method, you'd have to call the method non-virtually - which can happen with a call such as base.Foo(...)... but I'm not sure whether there's any way of making that sort of non-virtual call with a null argument. The delegate approach definitely doesn't work here.

Demo code:

using System;

class Test
{
    static void Main()
    {
        Action<Test> foo = (Action<Test>)
            Delegate.CreateDelegate(typeof(Action<Test>), typeof(Test).GetMethod("Foo"));
        foo(null); // Prints Weird
        Action<Test> bar = (Action<Test>)
            Delegate.CreateDelegate(typeof(Action<Test>), typeof(Test).GetMethod("Bar"));

        bar(null); // Throws
    }

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

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