Expression Tree Serializer

Question

I want to use Linq Expressions on Client Side, Serialize them and Execute them on server Side.

For this I want to use: http://expressiontree.codeplex.com/

But I want to Execute them agains a own WCF Call.

That means i Have a Call on WCf Side:

ImageDTO[] GetImages(XElement exp);

I now want to have a IQueryable on Client Side (on which I can execute Linq Expressions), and I have a IQueryable on Serverside (from my data acess layer, on wich i want to execute the serialized expression).

But I'm not sure how to do this, and I don't find any Examples...

On Client Side i think I should implement Query in a Class, this class I tell in the Constructor to use my implementation of QueryProvider (from where I call the WCF Service). But I'm not sure if this is correct...

Maybe someone can help with a Example.

Answer 1

There's an implementation of IQueryable<T> in the framework - MSDN: EnumerableQuery<T>

If you can use this on the client to build the query, you can get the whole Expression Tree from the IQueryable<T>.Expression property.

You'll have to test this to see if it works with that Expression Tree Serializer.

var iQueryable = new EnumerableQuery<Model>( Enumerable.Empty<Model>() );

var query = iQueryable.Include( ... ).Where( ... ).OrderBy( ... );

var expressionTree = query.Expression;

You can then serialize the expression, squirt it accross the wire and then deserialize it.

---

Then the problem is that the expression tree is based on an EnumerableQuery<T>.

So you need to replace that with your IQueryable<T> source from your real DbContext

This gets a bit messy, but I've written an implementation using an ExpressionVisitor:

IQueryable FixupExpressionTree( ObjectContext ctx, Type entityType, Expression expression )
{
    var tObjectContext = ctx.GetType();
    var mCreateObjectSetOpen = tObjectContext.GetMethod( "CreateObjectSet", new Type[ 0 ] );
    var mCreateObjectSetClosed = mCreateObjectSetOpen.MakeGenericMethod( entityType );

    var objectQuery = ( ObjectQuery ) mCreateObjectSetClosed.Invoke( ctx, null );

    var eFixed = new Visitor( objectQuery, entityType ).Visit( expression );

    var qFixed = ( ( IQueryable ) objectQuery ).Provider.CreateQuery( eFixed );

    return qFixed;
}

and the ExpressionVisitor itself:

public class Visitor : ExpressionVisitor
{
    ObjectQuery _Source = null;
    Type _EntityType = null;

    public Visitor( ObjectQuery source, Type entityType ) { _Source = source; _EntityType = entityType; }

    protected override Expression VisitConstant( ConstantExpression node )
    {
        if ( !node.Type.Name.Contains( "EnumerableQuery" ) ) return base.VisitConstant( node );

        var eConstantInstance = Expression.Constant( _Source );
        var eConstantArgument = Expression.Constant( MergeOption.AppendOnly );

        var tObjectQueryOpen = typeof( ObjectQuery<> );
        var tObjectQueryClosed = tObjectQueryOpen.MakeGenericType( _EntityType );
        var eMergeAsMethod = tObjectQueryClosed.GetMethod( "MergeAs", BindingFlags.Instance | BindingFlags.NonPublic );

        return Expression.Call( eConstantInstance, eMergeAsMethod, eConstantArgument );
    }
}

Calling this is straight forward:

Type entityType = ...
Expression expression = ...
DbContext db = ...

ObjectContext ctx = ( ( IObjectContextAdapter ) db ).ObjectContext;

IQueryable query = FixupExpressionTree( ctx, entityType, expression );

Answer 2

To Solve my Problem, I used Interlinq, but a Version wich I forked, so that it also works with Silverlight.

https://github.com/jogibear9988/Interlinq-2