What's the strangest corner case you've seen in C# or .NET?

here are a few of mine:

1. this can be null when calling an instance method with out a NullReferenceException being thrown
2. a default enumeration value doesn't have to be defined for the enumeration

Simple one first: enum NoZero { Number = 1 }

        public bool ReturnsFalse()
        {
            //The default value is not defined!
            return Enum.IsDefined(typeof (NoZero), default(NoZero));
        }

The below code can actually print true!

 internal sealed class Strange
{
    public void Foo()
    {
        Console.WriteLine(this == null);
    }
}

A simple piece of client code that will result in that is delegate void HelloDelegate(Strange bar);

public class Program
{
    [STAThread()]
    public static void Main(string[] args)
    {
        Strange bar = null;
        var hello = new DynamicMethod("ThisIsNull",
            typeof(void), new[] { typeof(Strange) },
         typeof(Strange).Module);
        ILGenerator il = hello.GetILGenerator(256);
        il.Emit(OpCodes.Ldarg_0);
        var foo = typeof(Strange).GetMethod("Foo");
        il.Emit(OpCodes.Call, foo);
        il.Emit(OpCodes.Ret);
        var print = (HelloDelegate)hello.CreateDelegate(typeof(HelloDelegate));
        print(bar);
        Console.ReadLine();
    }
}

this is actually true in most languages as long as the instance method when called doesn't use the state of the object. this is only dereferenced when the state of the object is accessed

This one had me truly puzzled (I apologise for the length but it's WinForm). I posted it in the newsgroups a while back.

I've come across an interesting bug. I have workarounds but i'd like to know the root of the problem. I've stripped it down into a short file and hope someone might have an idea about what's going on.

It's a simple program that loads a control onto a form and binds "Foo" against a combobox ("SelectedItem") for it's "Bar" property and a datetimepicker ("Value") for it's "DateTime" property. The DateTimePicker.Visible value is set to false. Once it's loaded up, select the combobox and then attempt to deselect it by selecting the checkbox. This is rendered impossible by the combobox retaining the focus, you cannot even close the form, such is it's grasp on the focus.

I have found three ways of fixing this problem.

a) Remove the binding to Bar (a bit obvious)

b) Remove the binding to DateTime

c) Make the DateTimePicker visible !?!

I'm currently running Win2k. And .NET 2.00, I think 1.1 has the same problem. Code is below.

using System;
using System.Collections;
using System.Windows.Forms;

namespace WindowsApplication6
{
    public class Bar
    {
        public Bar()
        {
        }
    }

    public class Foo
    {
        private Bar m_Bar = new Bar();
        private DateTime m_DateTime = DateTime.Now;

        public Foo()
        {
        }

        public Bar Bar
        {
            get
            {
                return m_Bar;
            }
            set
            {
                m_Bar = value;
            }
        }

        public DateTime DateTime
        {
            get
            {
                return m_DateTime;
            }
            set
            {