Partially executing an IQueryable query while leaving the remainder to Linq for Objects

c# expression-trees iqueryable linq


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 that possible? how?

3/30/2009 10:37:29 PM

Accepted Answer

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);
                return c;
4/28/2009 10:53:44 PM

Popular Answer

Both of the previous answers work, but it reads better if you use AsEnumerable() to cast the IQueryable to IEnumerable:

// Using Bob's code...
var result = datacontext.Table
   .Where(x => x.Prop == val)
   .OrderBy(x => x.Prop2)
   .AsEnumerable()  //  <---- anything after this is done by LINQ to Objects
   .Select(x => new { CoolProperty = x.Prop, OtherProperty = x.Prop2 });


// ... or MichaelGG's
var res = dc.Foos
           .Where(x => x.Bla > 0)  // uses IQueryable provider
           .Where(y => y.Snag > 0); // IEnumerable, uses LINQ to Objects

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