What s the strangest corner case you ve seen in C or NET

Question

I have a small collection of puzzles and corner cases, and I'm constantly interested in learning more. The page mainly deals with bits and pieces of the C# language, although I also find the fundamentals of.NET interesting. Here's one that isn't on the page but that I think is incredible:

string x = new string(new char[0]);
string y = new string(new char[0]);
Console.WriteLine(object.ReferenceEquals(x, y));

Since "new" (when used with a reference type) always creates a new object, I would expect it to print False. It should, according to the specifications for both C# and the CLI. Well, not in this specific instance. On every version of the framework I've tested it with, it prints True. (I must admit that I haven't tried it on Mono.)

Just to be clear, I wasn't specifically searching for a discussion or explanation of this peculiarity; this is merely an example of the kind of stuff I'm looking for. I was really looking for comparable strange behaviour (it's not the same as usual string interning; in particular, string interning doesn't typically happen when a function Object() { [native code] } is invoked.)

Any other gems lurking out there?

Answer 1

This took some debugging to track down! (the original code was obviously more complex and subtle...)

    static void Foo<T>() where T : new()
    {
        T t = new T();
        Console.WriteLine(t.ToString()); // works fine
        Console.WriteLine(t.GetHashCode()); // works fine
        Console.WriteLine(t.Equals(t)); // works fine

        // so it looks like an object and smells like an object...

        // but this throws a NullReferenceException...
        Console.WriteLine(t.GetType());
    }

So what was T...

 Any Nullable<T> - such as int? All the methods are overridden, except GetType() which can't be; so it is cast (boxed) to the object (and hence to null) to call the object.GetType()... which calls on null.