this == null inside .NET instance method - why is that possible?

Question

I\'ve always thought that it\'s impossible for this to be null inside instance method body. Following simple program demonstrates that it is possible. Is this s

Answer 1

Sure you can call into a method if you are using the call IL instruction or the delegate approach. You will set this booby trap only off if you try to access member fields which will give you the NullReferenceException you did seek for.

try

 int x;
 public void Bar()
 {
        x = 1; // NullRefException
        Debug.Assert(this == null);
 }

The BCL does even contain explicit this == null checks to aid debugging for languages which do not use callvirt (like C#) all the time. See this question for further infos.

The String class for example has such checks. There is nothing mysterious about them except that you will not see the need for them in languages like C#.

// Determines whether two strings match. 
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)] 
public override bool Equals(Object obj)
{
    //this is necessary to guard against reverse-pinvokes and
    //other callers who do not use the callvirt instruction
    if (this == null)
        throw new NullReferenceException();

    String str = obj as String;
    if (str == null) 
        return false;

    if (Object.ReferenceEquals(this, obj)) 
        return true;

    return EqualsHelper(this, str);
}