在运行时将身份（e => e）构造为表达式树

Question

I need to create identity expression tree at run-time that just returns the parameter that it receives.

At compile time I could use the lambda e => e as expression tree, but I need that transformation at run-time. I'm looking for code similar to how expression tree for num => num < 5 is constructed in MSDN Expression Trees article.

Answer 1

This will do it:

var param = Expression.Parameter(typeof(int), "e");
var body = param;
var lambda = Expression.Lambda<Func<int, int>>(body, param);

Output of lambda.ToString() :

e => e

Note that you need to give e a type (in this case it's an int).

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Pre-edit note:

What you've written ( e => e ) is actually already an expression tree (if defined as an Expression<T> , that is). However, it's likely you're wanting to use the factory methods to build the tree.

Answer 2

e => e . That's it.

It will need to be typed, of course, so eg Expression<Func<int, int>> intTree = e => e; for ints, Expression<Func<string, string>> stringTree = e => e; for strings, and so on.

Now you've got the tree from the lambda, you can examine it or do whatever else you want the tree for:

Console.WriteLine(intTree.Parameters[0].Name); // "e"
Console.WriteLine(intTree.CanReduce); // false
Func<int, int> intIdentityFunction = intTree.Compile();