Why can not be Func casted to Expression, but lambda can

Question

In the example below, the same lambda can be hold as an Expression in test but it fails to compile when it comes form a Func:

Func<int> func = () => 2;
Expression test1 = (Expression<Func<int>>)(() => 2);
Expression test2 = (Expression<Func<int>>)(func); //does not compile

Why a lambda can be converted to an Expression directly, but not when it is represented as a Func?

It is a compiler trick. The compiler parses the lambda and converts it into an expression. It cannot do that with a delegate. — Lasse V. Karlsen, Jul 10 '19 at 16:00
Answered over here: https://stackoverflow.com/questions/767733 — dezfowler, Jul 10 '19 at 16:00
@LasseVågsætherKarlsen by now, I like your comment the most, I think it clears things up in a simple manner. — meJustAndrew, Jul 10 '19 at 16:04

score 0 · Accepted Answer · answered Jul 10 '19 at 16:00

A Func<int> is a delegate instance - an instance of an object; it no longer has any semantic state other than "this target object reference, this method handle".

An expression tree is something much richer; it is a complex graph of the intent of code - essentially an AST. You can compile an expression to a delegate instance, but you cannot reverse a delegate to an expression tree.

When you do:

Expression<Func<int>> func = () => 2;

you are creating a complex tree; essentially:

var func = Expression.Lambda<Func<int>>(Expression.Constant(2, typeof(int)),
    Array.Empty<ParameterExpression>());

(but much more complex in the general case, obviously)

Why can not be Func casted to Expression, but lambda can

1 Answers1