How do I specify the object to return from an expression tree method?

Question

I'm trying to create a method using an expression tree that returns an object, but I can't figure out how to actually specify the object to return. I've tried reading this, but the return value doesn't actually seem to be specified anywhere.

I've got all the assignments & stuff down, but how do I specify the object to return from a method created using expression trees?

EDIT: these are v4 expression trees, and the method I'm trying to create does something like this:

private object ReadStruct(BinaryReader reader) {     StructType obj = new StructType();     obj.Field1 = reader.ReadSomething();     obj.Field2 = reader.ReadSomething();     //...more...     return obj; } 

Answer 1

There is a much easier way to do this in cases that return an existing parameter or variable. The last statement in a Block Expression becomes the return value. You can include the ParameterExpression again at the end to have it returned.

Assuming your struct is like this:

public struct StructType {     public byte Field1;     public short Field2; } 

Then your code would look like this:

var readerType = typeof(BinaryReader); var structType = typeof(StructType); var readerParam = Expression.Parameter(readerType); var structVar = Expression.Variable(structType);  var expressions = new List<Expression>();  expressions.Add(     Expression.Assign(         Expression.MakeMemberAccess(structVar, structType.GetField("Field1")),         Expression.Call(readerParam, readerType.GetMethod("ReadByte"))         )     );  expressions.Add(     Expression.Assign(         Expression.MakeMemberAccess(structVar, structType.GetField("Field2")),         Expression.Call(readerParam, readerType.GetMethod("ReadInt16"))         )     );  expressions.Add(structVar); //This is the key. This will be the return value.  var ReadStruct = Expression.Lambda<Func<BinaryReader, StructType>>(     Expression.Block(new[] {structVar}, expressions),     readerParam).Compile(); 

Test that it works:

var stream = new MemoryStream(new byte[] {0x57, 0x46, 0x07}); var reader = new BinaryReader(stream); var struct1 = ReadStruct(reader); 

It's worth mentioning this example works if StructType is a struct. If it is a class, you just call the constructor and initialize structVar first thing in the BlockExpression.

Answer 2

Apparently a return is a GotoExpression that you can create with the Expression.Return factory method. You need to create a label at the end to jump to it. Something like this:

// an expression representing the obj variable ParameterExpression objExpression = ...;  LabelTarget returnTarget = Expression.Label(typeof(StructType));  GotoExpression returnExpression = Expression.Return(returnTarget,      objExpression, typeof(StructType));  LabelExpression returnLabel = Expression.Label(returnTarget, defaultValue);  BlockExpression block = Expression.Block(     /* ... variables, all the other lines and... */,     returnExpression,     returnLabel); 

The types of the label target and the goto expression must match. Because the label target has a type, the label expression must have a default value.