Why is it not possible to evaluate lambdas in the immediate window?

Question

Is there any particular reason? Is it not possible at all or is it just not implemented yet? Maybe there are any third-party addins that allow lambda evaluations?

UPDATE:

I've found this project on codeplex Extended Immediate Window. Seems that it has been abandoned for some time, but this can be a proof of a concept. Does anybody know any other immediate window extension addins? The ones that can run for/foreach statements in C# for instance?

Answer 1

I assume, that because it is lazy evaluation, the immediate window cannot know beforehand what values, the captured variables (closure), should have.

Answer 2

Well, I think it's because the immediate window can only evaluate expressions, or rather it can only do invocations and assignments. To evaluate a Lambda expression a closure would have to be created for that lambda, typchecked and then executed.

I think it comes down to that the Immediate window is just an evaluator and not an interpreter.

http://msdn.microsoft.com/en-us/library/f177hahy(VS.80).aspx

"The Immediate window is used at design time to debug and evaluate expressions, execute statements, print variable values, and so forth. It allows you to enter expressions to be evaluated or executed by the development language during debugging."

So in effect, your question boils down to why you can't define functions in the immediate window (since lambdas are just annonymous functions), and the answer I think is that it simply wasn't designed for that.

Answer 3

When writing a lambda, the act of capturing variables significantly alters the construction of the underlying code (moving variables into fields of compiler-generated classes, that could very easily themselves be chained closure-contexts).

Not even considering the general complexity of doing this, it would then have two choices:

• capture all the variable values as constants; feasible and pretty simple, but could easily mean that the result of executing in the immediate window is very different to the result of executing in the main body (very undesirable)
• rewrite the entire code (for the reasons outlined above) on the fly (at a guess, impossible)

Given a choice between "undesirable" and "impossible", I guess they simply chose not to implement a feature that would be inherently brittle, and very complex to write.

Answer 4

JaredPar of Microsoft wrote a couple of blog posts answering your question: part 1 and part 2. You'll find the answers there.