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copied!<p>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...)</p> <pre><code> 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()); } </code></pre> <p>So what was T...</p> <p>Answer: any <code>Nullable<T></code> - such as <code>int?</code>. 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</p> <hr> <p>Update: the plot thickens... Ayende Rahien threw down a <a href="http://ayende.com/Blog/archive/2009/11/25/can-you-hack-this-out-hint-1.aspx" rel="nofollow noreferrer">similar challenge on his blog</a>, but with a <code>where T : class, new()</code>:</p> <pre><code>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?"); } </code></pre> <p>But it can be defeated! Using the same indirection used by things like remoting; warning - the following is <strong>pure evil</strong>:</p> <pre><code>class MyFunnyProxyAttribute : ProxyAttribute { public override MarshalByRefObject CreateInstance(Type serverType) { return null; } } [MyFunnyProxy] class MyFunnyType : ContextBoundObject { } </code></pre> <p>With this in place, the <code>new()</code> call is redirected to the proxy (<code>MyFunnyProxyAttribute</code>), which returns <code>null</code>. Now go and wash your eyes!</p>

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