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c#entity-frameworklinq-to-sqlexpression-treesiqueryable
dyatchenko
dyatchenko

Reputation: 2343

How to apply an expression tree for IQueryable?

Is there any way to apply a manually created expression tree from one IQueryable to another? For example:

IQueryable<string> p = Enumerable.Empty<string>().AsQueryable();
p = p.Where(pp => pp[0] == 'A');
p = p.Skip(2).Take(4);
p = p.OrderBy(pp => pp.Length);

var vv = new[] {"Afss", "Acv", "Adfv", "Bcvx", "Ng"}.AsQueryable();

// Filtering of the vv collection with expression from p 
var filteredResult = vv.Filter(p.Expression);

Or we have to separately apply the Where and OrderBy expressions?

Upvotes: 6

Views: 4853

Answers (3)

Vadim
Vadim

Reputation: 111

Simple CreateQuery call does not work, because provider uses data source of the passed expression.

Before executing expression you need to replace Enumerable.Empty<string>() by a collection with your data:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

public class Program
{
    public static void Main()
    {
        IQueryable<string> p = Enumerable.Empty<string>().AsQueryable();
        p = p.Where(pp => pp[0] == 'A');
        p = p.Skip(2).Take(4);
        p = p.OrderBy(pp => pp.Length);

        var vv = new[] {"Afss", "Acv", "Adfv", "Bcvx", "Ng"}.AsQueryable();

        // Filtering of the vv collection with expression from p 
        var expr = ExpressionTreeConstantReplacer.CopyAndReplace(p.Expression, typeof(EnumerableQuery<string>), vv);
        var filteredResult = vv.Provider.CreateQuery<string>(expr);

        Console.Write("Source: \t");
        foreach(var temp in vv)
            Console.Write("{0} ", temp);

        Console.WriteLine();

        Console.Write("Filtered: ");
        foreach(var temp in filteredResult)
            Console.WriteLine(temp);
    }

    class ExpressionTreeConstantReplacer<T> : ExpressionVisitor
    {
        Type originalType;
        T replacementConstant;

        internal ExpressionTreeConstantReplacer(Type originalType, T replacementConstant)
        {
            this.originalType = originalType;
            this.replacementConstant = replacementConstant;
        }

        protected override Expression VisitConstant(ConstantExpression c)
        {
            return c.Type == originalType ? Expression.Constant(replacementConstant) : c;
        }
    }

    class ExpressionTreeConstantReplacer
    {
        internal static Expression CopyAndReplace<T>(Expression expression, Type originalType, T replacementConstant)
        {
            var modifier = new ExpressionTreeConstantReplacer<T>(originalType, replacementConstant);
            return modifier.Visit(expression);
        }
    }
}

Upvotes: 6

Servy
Servy

Reputation: 203847

You can access the provider of the query and use CreateQuery to create a query based on an expression, and you can get the expression from the other query to pass to it:

var filteredResult = vv.Provider.CreateQuery<string>(p.Expression);

Upvotes: 3

Alioza
Alioza

Reputation: 1770

I had a similar case in which I had to send filter/sort expressions over WCF. I could not find a way to use a single instance for both the filter and the sort expression. I ended up using SerializableExpression from InterLinq. Doesn't solve your particular question but I hope it helps.

Upvotes: 0

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