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copied!<p>Recently I asked an almost identical question:</p> <p><a href="https://stackoverflow.com/questions/5053032/performance-of-compiled-to-delegate-expression">Performance of compiled-to-delegate Expression</a></p> <p>The solution for me was that I shouldn't call <code>Compile</code> on the <code>Expression</code>, but that I should call <code>CompileToMethod</code> on it and compile the <code>Expression</code> to a <code>static</code> method in a dynamic assembly. </p> <p>Like so:</p> <pre><code>var assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly( new AssemblyName("MyAssembly_" + Guid.NewGuid().ToString("N")), AssemblyBuilderAccess.Run); var moduleBuilder = assemblyBuilder.DefineDynamicModule("Module"); var typeBuilder = moduleBuilder.DefineType("MyType_" + Guid.NewGuid().ToString("N"), TypeAttributes.Public)); var methodBuilder = typeBuilder.DefineMethod("MyMethod", MethodAttributes.Public | MethodAttributes.Static); expression.CompileToMethod(methodBuilder); var resultingType = typeBuilder.CreateType(); var function = Delegate.CreateDelegate(expression.Type, resultingType.GetMethod("MyMethod")); </code></pre> <p>It's not ideal however. I'm not quite certain to which types this applies exactly, but I think that types that are taken as parameters by the delegate, or returned by the delegate <em>have</em> to be <code>public</code> and non-generic. It has to be non-generic because generic types apparently access <code>System.__Canon</code> which is an internal type used by .NET under the hood for generic types and this violates the "has to be a <code>public</code> type rule).</p> <p>For those types, you can use the apparently slower <code>Compile</code>. I detect them in the following way:</p> <pre><code>private static bool IsPublicType(Type t) { if ((!t.IsPublic && !t.IsNestedPublic) || t.IsGenericType) { return false; } int lastIndex = t.FullName.LastIndexOf('+'); if (lastIndex > 0) { var containgTypeName = t.FullName.Substring(0, lastIndex); var containingType = Type.GetType(containgTypeName + "," + t.Assembly); if (containingType != null) { return containingType.IsPublic; } return false; } else { return t.IsPublic; } } </code></pre> <p>But like I said, this isn't ideal and I would still like to know why compiling a method to a dynamic assembly is <em>sometimes</em> an order of magnitude faster. And I say sometimes because I've also seen cases where an <code>Expression</code> compiled with <code>Compile</code> is just as fast as a normal method. See my question for that. </p> <p>Or if someone knows a way to bypass the "no non-<code>public</code> types" constraint with the dynamic assembly, that's welcome as well.</p>

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