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POCan C# 'is' operator suffer under release mode optimization on .NET 4?
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  1. POCan C# 'is' operator suffer under release mode optimization on .NET 4?
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    copied!<p>Below is a simple test fixture. It succeeds in Debug builds and fails in Release builds (VS2010, .NET4 solution, x64):</p> <pre><code>[TestFixture] public sealed class Test { [Test] public void TestChecker() { var checker = new Checker(); Assert.That(checker.IsDateTime(DateTime.Now), Is.True); } } public class Checker { public bool IsDateTime(object o) { return o is DateTime; } } </code></pre> <p>It seems code optimization wreaks some havoc; if I disable it on the Release build, it works as well. That was rather puzzling to me. Below, I've used ILDASM to disassemble the 2 versions of the build:</p> <p>Debug IL:</p> <pre><code>.method public hidebysig instance bool IsDateTime(object o) cil managed { // Code size 15 (0xf) .maxstack 2 .locals init (bool V_0) IL_0000: nop IL_0001: ldarg.1 IL_0002: isinst [mscorlib]System.DateTime IL_0007: ldnull IL_0008: cgt.un IL_000a: stloc.0 IL_000b: br.s IL_000d IL_000d: ldloc.0 IL_000e: ret } // end of method Validator::IsValid </code></pre> <p>Release IL:</p> <pre><code>.method public hidebysig instance bool IsDateTime(object o) cil managed { // Code size 10 (0xa) .maxstack 8 IL_0000: ldarg.1 IL_0001: isinst [mscorlib]System.DateTime IL_0006: ldnull IL_0007: cgt.un IL_0009: ret } // end of method Validator::IsValid </code></pre> <p>It seems a store and load is optimized away. Targeting earlier versions of the .NET framework made the problem go away, but that may just be a fluke. I found this behaviour somewhat unnerving, can anybody explain why the compiler would think it safe to do an optimization that produces different observable behaviour?</p> <p>Thanks in advance.</p>
 

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