Linq Expression to execute existing expression on a child property

Question

Here is a simple class

public class Parent
{
    public Child Child { get; set; }
}

Here is the method I'm trying to implement

Expression<Func<Parent, long>> GetChildIDExpr(Expression<Func<Child, long>> objectIdExpr)
{
    //So I need to return an expression that can accept a Parent
    //and apply the given expression to its Child property.
}

Sounds simple enough, but I just can't work it out!

How can I implement this method?

EDIT:

I'm using LINQ to Entities so calling Expression.Invoke isn't going to work.

By execute an existing expression do you mean as a filter like 'parents.whereChild(express)'? — Master Morality, Jul 05 '12 at 18:52
Basically, Say `objectIdExpr = obj => obj.ID`, I need an entities way of doing `p => objectIdExpr.Invoke(p.Child)` — Connell, Jul 05 '12 at 18:55
check my answer, @SPFiredrake is correct, my answer is the same idea baked into a nice lib — Guillaume86, Jul 06 '12 at 17:41

score 1 · Answer 1 · edited May 23 '17 at 12:27

Ultimately it seems like you want to combine expressions. Here's the problem though: Expression trees are immutable, and they will maintain their parameter references which makes it hard to chain these together. However, that's not to say it can't be done (or isn't too difficult). You need the help of an ExpressionVisitor however, that will swap the parent and child parameter references.

class SwapVisitor : ExpressionVisitor
{
    private readonly Expression from, to;
    public SwapVisitor(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node)
    {
         return node == from ? to : base.Visit(node);
    }
}

Then you can just chain them together in your GetChildIDExpr:

Expression<Func<Parent, long>> GetChildIDExpr(Expression<Func<Child, long>> objectIdExpr)
{
    Expression<Func<Parent, Child>> parentEX = p => p.Child;
    var swap = new SwapVisitor(objectIdExpr.Parameters[0], parentEX.Body);
    var newExpr = Expression.Lambda<Func<Parent, long>>(
           swap.Visit(objectIdExpr.Body), parentEX.Parameters);
    return newExpr;
}

Give that a try and let us know.

This is a good solution but change 'swap.Visit(parentEX.Body), objectIdExpr.Parameters);' to 'swap.Visit(objectIdExpr.Body), parentEX.Parameters);' — Balazs Tihanyi, Jul 06 '12 at 08:56
Thanks @BalazsTihanyi, I had it right in my test project but not here. D'oh! — SPFiredrake, Jul 06 '12 at 14:36

score 1 · Answer 2 · answered Jul 06 '12 at 12:22

1

You can use LinqKit http://www.albahari.com/nutshell/linqkit.aspx with AsExpandable() and Compile()/Invoke(), it will inline external expressions, allowing use in Linq-to-sql or Linq-to-entities

answered Jul 06 '12 at 12:22

Guillaume86

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This is probably the safest way to do this, as it allows for proper composition of Expression trees. +1 – SPFiredrake Jul 06 '12 at 14:37

Linq Expression to execute existing expression on a child property

2 Answers2