Selecting from Lambda Expression with Lambda Expression

Question

I have two expressions:

  public static Expression<Func<TSource, TReturn>> Merge<TSource, TSource2, TReturn>(
      Expression<Func<TSource, TSource2>> foo1
      Expression<Func<TSource2, TReturn>> foo2)
  {
      // What to do?
  }

How can I merge them into a single expression, so the output from the first is used as the input for the second? I am new to these and so far its just been exceptions.

Thanks

can you add a concrete example that shows what you mean? is this as in x => x.Name and s => s.Length to get x => x.Name.Length ? — Marc Gravell
– Marc Gravell, Commented Mar 26, 2013 at 11:41
Yeah sorry that's what I meant, for future reference when people come to see this. — Tim
– Tim, Commented Mar 26, 2013 at 12:09

Marc Gravell · Accepted Answer · 2013-03-26 11:46:20Z

1

This depends a lot on which providers need to use it. Some will be fine with:

public static Expression<Func<TSource, TReturn>>
     Merge<TSource, TSource2, TReturn>(
  Expression<Func<TSource, TSource2>> foo1,
  Expression<Func<TSource2, TReturn>> foo2)
{
    return Expression.Lambda<Func<TSource, TReturn>>(
      Expression.Invoke(foo2, foo1.Body),
      foo1.Parameters);
}

However, others (EF) will not. You can also re-write the expression-tree with a visitor to inline the expression:

public static Expression<Func<TSource, TReturn>>
      Merge<TSource, TSource2, TReturn>(
  Expression<Func<TSource, TSource2>> foo1,
  Expression<Func<TSource2, TReturn>> foo2)
{
    var swapped = new SwapVisitor(
        foo2.Parameters.Single(), foo1.Body).Visit(foo2.Body);
    return Expression.Lambda<Func<TSource, TReturn>>(
        swapped, foo1.Parameters);
}

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);
    }
}

this will work with all providers.

answered Mar 26, 2013 at 11:46

Marc Gravell

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3 Comments

Tim Over a year ago

This indeed worked! Thanks I was extremely close, but I was using foo1, not foo1.Body in my overload of Invoke.

Marc Gravell Over a year ago

@Tim see the edit; the inlined version will usually be preferable, since it will work in more places (and can be more direct)

Tim Over a year ago

Also thanks for the extended answer. I am using LINQ-to-Entities so likely will be useful in the future. Edit: Ok thanks!

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