Why is code compiled with Expression<TDelegate>.Compile() slower than plain C#?

Question

I rewrote a method that used reflection with new code that uses the System.Linq.Expressions classes and the Expression.Compile() method.

As expected, the program is much faster then using reflection.

I also rewrote the same method in plain C# to compare and the code in C# is 4 times faster than the code compiled with Expression.Compile(). In my case, the method is called in a loop, thousands of times.

Note that I have taken out the 1st call from my profiling to make sure I don't measure the time to compile. So I compile the expression once and then call it thousand of times.

Why is code compiled with Expression.Compile() slower than plain C#?

Answer 1

It had already been noted that you must cache and re-use the delegate, ideally ignoring the first run. To give a complete answer we would need to see a specific example. I've actually seen examples where it was faster (due to different IL flags being set).

So: it will depend on the code. My guess is that your example does some conversion or operator that the c# compiler handles differently (optimised), but Expression must handle in a generic way (since it is language independent).

Another factor is how things like literals and captures are handled.