Question

I have a Linq provider that sucessfully goes and gets data from my chosen datasource, but what I would like to do now that I have my filtered resultset, is allow Linq to Objects to process the rest of the Expression tree (for things like Joins, projection etc)

My thought was that I could just replace the expression constant that contains my IQueryProvider with the result-sets IEnumerable via an ExpressionVisitor and then return that new expression. Also return the IEnumerable’s provider from my IQueryable…but this does not seem to work 🙁

Any idea’s?

Edit:
Some good answers here, but given the form…

var qry = from c in MyProv.Table<Customer>()
          Join o in MyProv.Table<Order>() on c.OrderID equals o.ID
          select new 
          {
            CustID = c.ID,
            OrderID = o.ID
          }

In my provider I can easily get back the 2 resultsets from Customers and Orders, if the data was from a SQL source I would just construct and pass on the SQL Join syntax, but it this case the data is not from a SQL source so I need to do the join in code…but as I said I have the 2 result sets, and Linq to Objects can do a join…(and later the projection) it would be real nice to just substitute the Expression constants MyProv.Table<Customer> and MyProv.Table<Order> with List<Customer> and List<Order> and let a List<> provider process the expression…is that possible? how?

Answer 1

The kind of thing that I was after was replacing the Queryable<> constant in the expression tree with a concrete IEnumerable (or IQueryable via .AsQueryable()) result set…this is a complex topic that probably only makes any sense to Linq Provider writers who are knee deep in expression tree visitors etc.

I found a snippet on the msdn walkthrough that does something like what I am after, this gives me a way forward…

using System;
using System.Linq;
using System.Linq.Expressions;

namespace LinqToTerraServerProvider
{
    internal class ExpressionTreeModifier : ExpressionVisitor
    {
        private IQueryable<Place> queryablePlaces;

        internal ExpressionTreeModifier(IQueryable<Place> places)
        {
            this.queryablePlaces = places;
        }

        internal Expression CopyAndModify(Expression expression)
        {
            return this.Visit(expression);
        }

        protected override Expression VisitConstant(ConstantExpression c)
        {
            // Replace the constant QueryableTerraServerData arg with the queryable Place collection.
            if (c.Type == typeof(QueryableTerraServerData<Place>))
                return Expression.Constant(this.queryablePlaces);
            else
                return c;
        }
    }
}