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Posted 18 Mar 2008

Modifying LINQ Expressions with Rewrite Rules

, 18 Mar 2008
Rewriting query expressions is a simple and yet safe and powerful technique to modify queries dynamically at runtime.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Windows.Forms;

using Rewrite;
using RewriteSql;
using System.Data.Linq;

namespace RewriteDemo {
    public class CustomerExt {
        public string Address { get; set; }
        public string City { get; set; }
        public string CompanyName { get; set; }
        public string Country { get; set; }
        public string CustomerID { get; set; }
        public EntitySet<Order> Orders { get; set; }
        public string Region { get; set; }

        public int OrdersNo { get; set; }
    }

    public class DataAccess {
        public static string TheConnectString {
            get { return RewriteDemo.Properties.Settings.Default.NORTHWNDConnectionString; }
        }

        public static List<CustomerExt> GetCustomersSorted(
            Expression<Func<Customer, bool>> filter,
            Expression<Func<IQueryable<CustomerExt>, IOrderedQueryable<CustomerExt>>> orderByClause, 
            Action<string> showQuery ) 
        {
            try {
                using( NWindDataContext ctx = new NWindDataContext( TheConnectString ) ) {
                    Func<Customer, bool> dummyFilter = null;
                    Func<CustomerExt, object> dummySelector = null;

                    var q = from ext in
                                ( from c in ctx.Customers
                                  where dummyFilter( c )
                                  select new CustomerExt
                                  {
                                      OrdersNo = c.Orders.Count(),
                                      Address = c.Address,
                                      City = c.City,
                                      CompanyName = c.CompanyName,
                                      Country = c.Country,
                                      CustomerID = c.CustomerID,
                                      Orders = c.Orders,
                                      Region = c.Region
                                  } )
                            orderby dummySelector( ext )
                            select ext
                            ;

                    var e1 = WhereRewriter.Rewrite( q.Expression, x => dummyFilter( x ), filter );
                    var e2 = OrderByRewriter.Rewrite( e1, x => dummySelector( x ), orderByClause );

                    showQuery( e2.ToString() );

                    return WhereRewriter.RecreateQuery( q, e2 ).ToList();
                }
            }
            catch( Exception ex ) {
                MessageBox.Show( "Error in GetCustomersSorted(): " + ex.Message );
                return null;
            }
        }

    }
}

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About the Author

Dmitri Raiko
Germany Germany
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