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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.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Windows.Forms;
using System.Reflection;

using Rewrite;
using RewriteSql;

namespace RewriteDemo {
    public partial class Form1: Form {
        public Form1() {
            InitializeComponent();
            this.FillCompareOpCombos();
            this.FillOrderByCombos();
        }

        private void FillOrderByCombos() {
            this.FillOrderByCombo( this.cboSort1 );
            this.FillOrderByCombo( this.cboSort2 );
        }

        private void FillOrderByCombo( ComboBox comboBox ) {
            comboBox.Items.Clear();
            comboBox.Items.AddRange( UserInput.CustomerExtProps );
        }

        private void FillCompareOpCombos() {
            this.FillYearCombo();
            this.FillPropCombo( this.cboWhereProp1 );
            this.FillPropCombo( this.cboWhereProp2 );
        }

        private void FillPropCombo( ComboBox comboBox ) {
            comboBox.Items.Clear();
            comboBox.Items.AddRange( UserInput.CustomerProps );
        }

        private void FillYearCombo() {
            this.cboYears.Items.Clear();
            this.cboYears.Items.Add( "" );
            this.cboYears.Items.AddRange( new object[] { 1996, 1997, 1998 } );
        }

        private void ShowWhereCond( string txt ) {
            StringBuilder sb = new StringBuilder( txt );
            sb.Replace( ".Where(", "\r\n.Where(" );
            sb.Replace( ".Select(", "\r\n.Select(" );
            sb.Replace( ".OrderBy", "\r\n.OrderBy" );
            
            this.txtWhereQuery.Text = sb.ToString();
        }

        private void btnSortRead_Click( object sender, EventArgs e ) {
            IList<CustomerExt> lst = DataAccess.GetCustomersSorted( 
                this.MakeCustomerFilter(), 
                this.MakeCustomerOrderBy(), 
                this.ShowWhereCond );
            if( lst != null ) {
                BindingList<CustomerExt> blc = new BindingList<CustomerExt>( lst );
                this.customerBindingSource.DataSource = blc;
            }
        }

        private Expression<Func<IQueryable<CustomerExt>, IOrderedQueryable<CustomerExt>>> MakeCustomerOrderBy() {
            IOrderByItem<CustomerExt> skey1 = null;
            if( !string.IsNullOrEmpty( this.cboSort1.Text ))
                skey1 = OrderByBuilder<CustomerExt>.BuildGeneric( this.cboSort1.Text, this.chkSortDir1.Checked);
            IOrderByItem<CustomerExt> skey2 = null;
            if( !string.IsNullOrEmpty( this.cboSort2.Text ))
                skey2 = OrderByBuilder<CustomerExt>.BuildGeneric( this.cboSort2.Text, this.chkSortDir2.Checked);

            OrderByItem<CustomerExt,int> skey3 = null;
            if( !string.IsNullOrEmpty( this.cboSortYear.Text ) ) {
                int year;
                if( int.TryParse( this.cboSortYear.Text, out year ) ) {
                    Expression<Func<CustomerExt, int>> expr3 = 
                        z => z.Orders.Count( o => o.OrderDate.HasValue && o.OrderDate.Value.Year == year );
                    // for better readability:
                    expr3 = SimpleRewriter.ApplyOnce( expr3,
                            Rewrite.Rule.Create( () => year, EvaluateLiteral.CreateRhs( year ) ) );
                    skey3 = new OrderByItem<CustomerExt, int>(
                        expr3,
                        true );
                }
            }

            return OrderByBuilder<CustomerExt>.MakeOrderByClause( skey1, skey2, skey3 );
        }

        private Expression<Func<Customer, bool>> MakeCustomerFilter() {
            Expression<Func<Customer, bool>> cityFilter = UserInput.MakeCityFilter( this.txtWhereCity.Text );
            Expression<Func<Customer, bool>> ordersInYearFilter
                = UserInput.MakeOrdersInYearFilter( this.cboYears.Text, this.txtWhereOrderNo.Text );
            Expression<Func<Customer, bool>> generic1 = UserInput.MakeGerericFilter( this.cboWhereProp1.Text, this.cboWhereCompareOp1.Text, this.txtWhereValue1.Text );
            Expression<Func<Customer, bool>> generic2 = UserInput.MakeGerericFilter( this.cboWhereProp2.Text, this.cboWhereCompareOp2.Text, this.txtWhereValue2.Text );

            Expression<Func<Customer, bool>> ret;
            if( this.rbAnd.Checked )
                ret = FilterBuilder.And( generic1, generic2 );
            else
                ret = FilterBuilder.Or( generic1, generic2 );
            if( this.chkNot.Checked )
                ret = FilterBuilder.Not( ret );
            return FilterBuilder.And( ret, cityFilter, ordersInYearFilter );
        }

        private void cboWhereProp2_SelectedIndexChanged( object sender, EventArgs e ) {
            PropertyInfo pi = this.cboWhereProp2.SelectedItem as PropertyInfo;
            if( pi == null)
                return; // -------------->>>>>>>>>>>>>>>>>>>>>
            this.RefreshCompareOpCbo( pi, this.cboWhereCompareOp2 );
        }

        private void RefreshCompareOpCbo( PropertyInfo pi, ComboBox opCbo ) {
            string txt = (string)opCbo.SelectedItem;
            opCbo.Items.Clear();
            opCbo.Text = null;
            var items = UserInput.CompareOps( pi.PropertyType );
            if( items != null )
                opCbo.Items.AddRange( items);
            opCbo.SelectedItem = txt;
        }

        private void cboWhereProp1_SelectedIndexChanged( object sender, EventArgs e ) {
            PropertyInfo pi = this.cboWhereProp1.SelectedItem as PropertyInfo;
            if( pi == null )
                return; // -------------->>>>>>>>>>>>>>>>>>>>>
            this.RefreshCompareOpCbo( pi, this.cboWhereCompareOp1 );
        }
    }
}

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About the Author

Dmitri Raiko

Germany Germany
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