Compiled C# Lambda Expressions Performance

Question

Could it be that the inner lambdas are not being compiled?!? Here’s a proof of concept:

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);
    }

And now the timings are:

Using lambda:                            625020
Using lambda compiled:                 14687970
Using lambda combined:                   468765
Using lambda compiled with MethodCall:   468765

Woot! Not only it is fast, it is faster than the native lambda. (Scratch head).

Of course the above code is simply too painful to write. Let’s do some simple magic:

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);
}

And some timings, VS2010, Optimizations ON, Debugging OFF:

Using lambda:                            781260
Using lambda compiled:                 14687970
Using lambda combined:                   468756
Using lambda compiled with MethodCall:   468756
Using lambda compiled constant:          468756

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.

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 CreateDelegate. But to truly understand this, I would love to have a .NET guru comment a little about the internal stuff going on.

And why, oh why is this now faster than a native lambda!?

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