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    <p>This one's pretty hard to top. I ran into it while I was trying to build a RealProxy implementation that truly supports Begin/EndInvoke (thanks MS for making this impossible to do without horrible hacks). This example is basically a bug in the CLR, the unmanaged code path for BeginInvoke doesn't validate that the return message from RealProxy.PrivateInvoke (and my Invoke override) is returning an instance of an IAsyncResult. Once it's returned, the CLR gets incredibly confused and loses any idea of whats going on, as demonstrated by the tests at the bottom.</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Runtime.Remoting.Proxies; using System.Reflection; using System.Runtime.Remoting.Messaging; namespace BrokenProxy { class NotAnIAsyncResult { public string SomeProperty { get; set; } } class BrokenProxy : RealProxy { private void HackFlags() { var flagsField = typeof(RealProxy).GetField("_flags", BindingFlags.NonPublic | BindingFlags.Instance); int val = (int)flagsField.GetValue(this); val |= 1; // 1 = RemotingProxy, check out System.Runtime.Remoting.Proxies.RealProxyFlags flagsField.SetValue(this, val); } public BrokenProxy(Type t) : base(t) { HackFlags(); } public override IMessage Invoke(IMessage msg) { var naiar = new NotAnIAsyncResult(); naiar.SomeProperty = "o noes"; return new ReturnMessage(naiar, null, 0, null, (IMethodCallMessage)msg); } } interface IRandomInterface { int DoSomething(); } class Program { static void Main(string[] args) { BrokenProxy bp = new BrokenProxy(typeof(IRandomInterface)); var instance = (IRandomInterface)bp.GetTransparentProxy(); Func&lt;int&gt; doSomethingDelegate = instance.DoSomething; IAsyncResult notAnIAsyncResult = doSomethingDelegate.BeginInvoke(null, null); var interfaces = notAnIAsyncResult.GetType().GetInterfaces(); Console.WriteLine(!interfaces.Any() ? "No interfaces on notAnIAsyncResult" : "Interfaces"); Console.WriteLine(notAnIAsyncResult is IAsyncResult); // Should be false, is it?! Console.WriteLine(((NotAnIAsyncResult)notAnIAsyncResult).SomeProperty); Console.WriteLine(((IAsyncResult)notAnIAsyncResult).IsCompleted); // No way this works. } } } </code></pre> <p>Output:</p> <pre><code>No interfaces on notAnIAsyncResult True o noes Unhandled Exception: System.EntryPointNotFoundException: Entry point was not found. at System.IAsyncResult.get_IsCompleted() at BrokenProxy.Program.Main(String[] args) </code></pre>
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    1. POWhat's the strangest corner case you've seen in C# or .NET?
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