What’s the strangest corner case you’ve seen in C# or .NET? [closed]

I think I showed you this one before, but I like the fun here – 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…

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

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Update: the plot thickens… Ayende Rahien threw down a similar challenge on his blog, but with a where T : class, new():

private static void Main() {
    CanThisHappen<MyFunnyType>();
}

public static void CanThisHappen<T>() where T : class, new() {
    var instance = new T(); // new() on a ref-type; should be non-null, then
    Debug.Assert(instance != null, "How did we break the CLR?");
}

But it can be defeated! Using the same indirection used by things like remoting; warning – the following is pure evil:

class MyFunnyProxyAttribute : ProxyAttribute {
    public override MarshalByRefObject CreateInstance(Type serverType) {
        return null;
    }
}
[MyFunnyProxy]
class MyFunnyType : ContextBoundObject { }

With this in place, the new() call is redirected to the proxy (MyFunnyProxyAttribute), which returns null. Now go and wash your eyes!