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We can write below code:

 Func<string, string> func = x => x + x;

We also can write:

 Expression<Func<string, string>> exp = x => x + x;

But when I write :

Expression<Func<string, string>> exp = func;

The compiler throw an error:

Cannot implicitly convert type 'System.Func' to 'System.Linq.Expressions.Expression>'

So I change the code as below:

  Expression<Func<string, string>> exp = (Expression<Func<string, string>>)func;

Same error as before.

So what's the real type of x => x + x; , what's the relation between Expression and Delegate/Lambda Expression?

Flat Eric
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roast_soul
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1 Answers1

6

So what's the real type of x => x + x;

It doesn't have one. A lambda expression is implicitly convertible into compatible delegate types and expression tree types (with some restrictions) but that decision is made at compile-time, and different code is generated depending on what the target of the conversion is.

You can convert from an expression tree to a delegate at execution time (using LambdaExpression.Compile) but you can't go the other way.

Basically, a lambda expression is a source representation of some logic. The compiler can either generate a code representation of that logic (conversion to delegate) or a data representation of that logic (conversion to expression tree). To be very specific, for expression trees, code is generated that will build the data representation.

Jon Skeet
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  • I'm not sure this truly answers this question. You say at compile-time, but the type of func is known at compile time. – Lews Therin May 05 '16 at 10:54
  • @LewsTherin: Yes, it is. But that's not the type of the lambda expression itself - that doesn't have a type. (Just like the `null` literal doesn't have a type, and anonymous methods, and method groups.) – Jon Skeet May 05 '16 at 10:55