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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.Text;
using System.Reflection;
using Rewrite;
using System.Linq.Expressions;
using System.Data.Linq.SqlClient;

using RewriteSql;

namespace RewriteDemo {
    class UserInput {
        internal static PropertyInfo[] CustomerProps {
            get {
                // exclude EntitySets
                return typeof( Customer ).GetProperties().Where( p => !p.PropertyType.IsGenericType ).ToArray();
            }
        }

        internal static PropertyInfo[] CustomerExtProps {
            // exclude EntitySets
            get { return typeof( CustomerExt ).GetProperties().Where( p => !p.PropertyType.IsGenericType).ToArray(); }
        }

        internal static string[] CompareOps( Type type ) {
            IEnumerable<string> ops = CompareOpDecoder.GetCompareOpNames( type );
            if( ops != null )
                return ops.ToArray();
            return null;
        }

        public static Expression<Func<Customer, bool>> MakeCityFilter( string value ) {
            if( string.IsNullOrEmpty( value ) )
                return null; // ---------->>>>>>>>>>>>>
            value = value.ToUpper();

            Expression<Func<Customer, bool>> expr = c => SqlMethods.Like( c.City.ToUpper(), value );

            // this is done solely for better readability
            expr = SimpleRewriter.ApplyOnce( expr,
                Rule.Create( () => value, EvaluateLiteral.CreateRhs( value ) ) );

            return expr;
        }

        public static Expression<Func<Customer, bool>> MakeOrdersInYearFilter( string yearStr, string countStr ) {
            int cnt, year;
            if( !int.TryParse( countStr, out cnt ) || !int.TryParse( yearStr, out year ) ) 
                return null; // ---------->>>>>>>>>>>>>

            Expression<Func<Customer, bool>> expr 
                = c => c.Orders.Count( o => o.OrderDate.HasValue && o.OrderDate.Value.Year == year ) >= cnt;
            var rwr = new SimpleRewriter( expr );
            Expression<Func<int>> yearLhs = () => year;
            rwr.ApplyOnce( new Rule( yearLhs, EvaluateLiteral.CreateRhs( typeof( int ), year ) ) );
            Expression<Func<int>> cntLhs = () => cnt;
            rwr.ApplyOnce( new Rule( cntLhs, EvaluateLiteral.CreateRhs( typeof( int ), cnt ) ) );
            
            return (Expression<Func<Customer, bool>>)rwr.Expression;
        }

        public static Expression<Func<Customer, bool>> MakeGerericFilter( string propName, string opName, string value ) {
            if( string.IsNullOrEmpty( propName) || string.IsNullOrEmpty( opName) || string.IsNullOrEmpty( value))
                return null; // ---------->>>>>>>>>>>>>

            return GenericBinFilter<Customer>.Create( propName, opName, value );
        }
    }
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Dmitri Raiko
Germany Germany
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