What is the max LINQ Expression Trees can do?

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+1 Q:

What is the max LINQ Expression Trees can do?

What is the maximun that LINQ expression Tree can do?

Can it define a class? How about a method, with all the declared name, modifiers, parametertype and return type?

Must the program always define the tree himself? Is it possible to generate the tree from a given C# file?

Where can I get resources to learn about writing basic to advanced Expression Tree and Expression Tree Visitor? (articles and videos will be great)

Thanks for those who are able to help...

+11 A:

In C# 3, expression trees can represent expressions. Hence the name. And they are further restricted to a subset of C# expressions -- no assignment expressions, no expressions involving pointer types, and so on.

In the libraries that will ship with C# 4, we have extended the expression tree library to also support statement trees. However, C# 4 will NOT automatically translate a statement-lambda into a "statement tree".

That is an obvious and useful feature which we simply did not have time to get to for C# 4. We'll consider it for hypothetical future versions. If you have a really great user scenario for statement trees, I'd love to hear it.

The obvious further extension to all that is declaration trees, which would represent class declarations, struct declarations, and so on. Having total homoiconicity between the C# language and the expression tree library would be awesome. It would enable all kinds of interesting metaprogramming scenarios. But that will not happen any time soon, so do not get your hopes up. That's more of a long-term dream of mine which might never happen.

Eric Lippert 2009-09-16 15:18:42

Thanks for ur reply. This is the library of Expression: http://msdn.microsoft.com/en-us/library/system.linq.expressions(VS.100).aspx. But I see MemberAssignment there, it is not for assignment expression? And this MethodCallExpression, shouldn't it be able to extract info from a method call (callee/caller/parameters)?

yeeen 2009-09-17 07:10:10

No, that is not what MemberAssignment is for. I do not understand your second question.

Eric Lippert 2009-09-17 13:38:38

Then what is MemberAssignment for? I mean that is it able to extract the details of an method invocation (callee, caller, parameter types)? Or is it even possible to define a method and call another method using Expression?

yeeen 2009-09-17 16:43:27

MemberAssignment is for representing the notion "initialize the members of this newly-created object". It's how we represent initialization of an anonymous type instance in an expression tree. But rather than asking me and waiting for my response, why not just read the documentation that you linked to? That's probably a faster way to learn about expression trees.

Eric Lippert 2009-09-17 19:31:45

Hi Eric, you said you'd love to hear if there were any great user scenarios for statement trees. I have one: http://stackoverflow.com/questions/3988560/how-can-i-convert-a-predicatet-to-an-expressionpredicatet-to-use-with-moq/3988581#3988581 - just got pointed to this comment and thought you might like to know. Working around it with a delegate method for the moment but it doesn't read as clearly. Thanks!

Lunivore 2010-10-21 15:09:27

@Lunivore: Thanks!

Eric Lippert 2010-10-21 21:41:51

+1 A:

Expression trees:

http://www.interact-sw.co.uk/iangblog/2005/09/30/expressiontrees
http://www.davidhayden.com/blog/dave/archive/2006/12/18/ExpressionTrees.aspx
http://msdn.microsoft.com/en-us/library/bb397951.aspx
http://community.bartdesmet.net/blogs/bart/archive/2006/11/22/Getting-started-with-C_2300_-3.0-Expression-Trees.aspx
http://blah.winsmarts.com/2006/05/23/demystifying-c-30--part-7-expression-trees.aspx

Visitor pattern:

http://www.dofactory.com/Patterns/PatternVisitor.aspx
http://en.wikipedia.org/wiki/Visitor_pattern

flesh 2009-09-16 20:02:32

ansaurus

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What is the max LINQ Expression Trees can do?

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