Lambda to Expression tree conversion

You must assign the lambda to a different type:

// Gives you a delegate:
Func<int, int> f = x => x * 2;
// Gives you an expression tree:
Expression<Func<int, int>> g = x => x * 2;

The same goes for method arguments. However, once you've assigned such a lambda expression to a Func<> type, you can't get the expression tree back.

Konrad's reply is exact. You need to assign the lambda expression to Expression<Func<...>> in order for the compiler to generate the expression tree. If you get a lambda as a Func<...>, Action<...> or other delegate type, all you have is a bunch of IL instructions.

If you really need to be able to convert an IL-compiled lambda back into an expression tree, you'd have to decompile it (e.g. do what Lutz Roeder's Reflector tool does). I'd suggest having a look at the Cecil library, which provides advanced IL manipulation support and could save you quite some time.

Just to expand on Konrad's answer, and to correct Pierre, you can still generate an Expression from an IL-compiled lambda, though it's not terribly elegant. Augmenting Konrad's example:

// Gives you a lambda:
Func<int, int> f = x => x * 2;

// Gives you an expression tree:
Expression<Func<int, int>> g = x => f(x);