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copied!<p>Could it be that the inner lambdas are not being compiled?!? Here's a proof of concept:</p> <pre><code>static void UsingCompiledExpressionWithMethodCall() { var where = typeof(Enumerable).GetMember("Where").First() as System.Reflection.MethodInfo; where = where.MakeGenericMethod(typeof(int)); var l = Expression.Parameter(typeof(IEnumerable<int>), "l"); var arg0 = Expression.Parameter(typeof(int), "i"); var lambda0 = Expression.Lambda<Func<int, bool>>( Expression.Equal(Expression.Modulo(arg0, Expression.Constant(2)), Expression.Constant(0)), arg0).Compile(); var c1 = Expression.Call(where, l, Expression.Constant(lambda0)); var arg1 = Expression.Parameter(typeof(int), "i"); var lambda1 = Expression.Lambda<Func<int, bool>>(Expression.GreaterThan(arg1, Expression.Constant(5)), arg1).Compile(); var c2 = Expression.Call(where, c1, Expression.Constant(lambda1)); var f = Expression.Lambda<Func<IEnumerable<int>, IEnumerable<int>>>(c2, l); var c3 = f.Compile(); var t0 = DateTime.Now.Ticks; for (int j = 1; j < MAX; j++) { var sss = c3(x).ToList(); } var tn = DateTime.Now.Ticks; Console.WriteLine("Using lambda compiled with MethodCall: {0}", tn - t0); } </code></pre> <p>And now the timings are:</p> <pre><code>Using lambda: 625020 Using lambda compiled: 14687970 Using lambda combined: 468765 Using lambda compiled with MethodCall: 468765 </code></pre> <p>Woot! Not only it is fast, it is faster than the native lambda. (<em>Scratch head</em>).</p> <hr> <p>Of course the above code is simply too painful to write. Let's do some simple magic:</p> <pre><code>static void UsingCompiledConstantExpressions() { var f1 = (Func<IEnumerable<int>, IEnumerable<int>>)(l => l.Where(i => i % 2 == 0)); var f2 = (Func<IEnumerable<int>, IEnumerable<int>>)(l => l.Where(i => i > 5)); var argX = Expression.Parameter(typeof(IEnumerable<int>), "x"); var f3 = Expression.Invoke(Expression.Constant(f2), Expression.Invoke(Expression.Constant(f1), argX)); var f = Expression.Lambda<Func<IEnumerable<int>, IEnumerable<int>>>(f3, argX); var c3 = f.Compile(); var t0 = DateTime.Now.Ticks; for (int j = 1; j < MAX; j++) { var sss = c3(x).ToList(); } var tn = DateTime.Now.Ticks; Console.WriteLine("Using lambda compiled constant: {0}", tn - t0); } </code></pre> <p>And some timings, VS2010, Optimizations ON, Debugging OFF:</p> <pre><code>Using lambda: 781260 Using lambda compiled: 14687970 Using lambda combined: 468756 Using lambda compiled with MethodCall: 468756 Using lambda compiled constant: 468756 </code></pre> <p>Now you could argue that I'm not generating the whole expression dynamically; just the chaining invocations. But in the above example I generate the whole expression. And the timings match. This is just a shortcut to write less code.</p> <hr> <p>From my understanding, what is going on is that the .Compile() method does not propagate the compilations to inner lambdas, and thus the constant invocation of <code>CreateDelegate</code>. But to truly understand this, I would love to have a .NET guru comment a little about the internal stuff going on.</p> <p>And <em>why</em>, oh <em>why</em> is this now faster than a native lambda!?</p>

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