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LINQ and Dynamic Predicate Construction at Runtime

, 13 Aug 2008
Illustrating a multi-predicate injection pattern now possible with the new features of C# 3.0.
LINQDynamicPredicate_demo.zip
LINQDynamicSearchDemo
App_Data
Global.asax
NorthwindDB.dbml
NorthwindDB.dbml.layout
Properties
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace LINQDynamicSearchDemo {
    public partial class _Default : System.Web.UI.Page {
        protected void Page_Load(object sender, EventArgs e) {

        }

        void BindTheQuery() {
            
            // I have this separated in case you want to do paging, we still delay the execution of the query. 
            // That way you can tack on some more expressions before executing it. In this example, I don't do that.

            ResultGrid.DataSource = PrepareDataSource();

        }

        void BindTheData() {

            BindTheQuery();

            /* 
             
             last chance to add stuff to the query. If you do it here, you'll have to cast to IQueryable<Employee>,
             so perhaps do it in BindTheQuery() or PrepareDataSource()
             
             */

            // finally execute the query.
            ResultGrid.DataBind();
        }

        public IQueryable<Employee> PrepareDataSource() {
            
            var predicate = PredicateBuilder.True<Employee>();

            // we start with true, as in, no filters, get all of them.


            // just check the 'checks'... throw in some lambdas

            int emplId = -1;

            // use tryparse to make sure we don't run a bogus query.
            if (cbxUseEmployeeID.Checked &&
                int.TryParse(filterEmployeeId.Text, out emplId) &&
                emplId > 0) {

                // here's how simple it is to add a condition to the query. Still not executing yet, just building a tree.
                predicate = predicate.And(e => e.EmployeeID == emplId);
            }

            if (cbxUseLastName.Checked &&
                !string.IsNullOrEmpty(filterLastName.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.LastName.Contains(filterLastName.Text));
            }

            if (cbxUseFirstName.Checked &&
                !string.IsNullOrEmpty(filterFirstName.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.FirstName.Contains(filterFirstName.Text));
            }

            if (cbxUseTitle.Checked &&
                !string.IsNullOrEmpty(filterTitle.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.Title.Contains(filterTitle.Text));
            }

            DateTime startDateRange = new DateTime(); // default value to avoid 'unassigned use' errors. 
            DateTime endDateRange = new DateTime(); 

            if (cbxUseBirthDate.Checked &&
                DateTime.TryParse(filterBirthDateStart.Text, out startDateRange) &&
                DateTime.TryParse(filterBirthDateEnd.Text, out endDateRange)) {

                // tack on some numeric range testing. I'd have liked to do a between query, but I don't know if
                // there is one that translates in such a way, so we'll just do this:

                predicate = predicate.And(e => e.BirthDate.Value >= startDateRange && e.BirthDate.Value <= endDateRange);
            }

            if (cbxUseAddress.Checked &&
                !string.IsNullOrEmpty(filterAddress.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.Address.Contains(filterAddress.Text));
            }

            if (cbxUseCity.Checked &&
                !string.IsNullOrEmpty(filterCity.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.City.Contains(filterCity.Text));
            }

            if (cbxUseState.Checked &&
                !string.IsNullOrEmpty(filterState.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.Region.Contains(filterState.Text));
            }
            

            if (cbxUsePostalCode.Checked &&
                !string.IsNullOrEmpty(filterPostalCode.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.PostalCode.Contains(filterPostalCode.Text));
            }

            if (cbxUseCountry.Checked &&
                !string.IsNullOrEmpty(filterCountry.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.Country.Contains(filterCountry.Text));
            }

            if (cbxUseHomePhone.Checked &&
                !string.IsNullOrEmpty(filterHomePhone.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.HomePhone.Contains(filterHomePhone.Text));
            }

            if (cbxUseNotes.Checked &&
                !string.IsNullOrEmpty(filterNotes.Text)) {

                // this translates into a LIKE query.
                predicate = predicate.And(e => e.Notes.Contains(filterNotes.Text));
            }

            var results = Config.GetCurrentContext().Employees.Where(predicate);

            // If you are debugging, you can put a breakpoint up there and see the Query by hovering over 'results'.

            return results;
        }

        protected void searchButton_Click(object sender, EventArgs e) {
            BindTheData();
        }

        protected void ResultGrid_PageIndexChanging(object sender, GridViewPageEventArgs e) {
            ResultGrid.PageIndex = e.NewPageIndex;
            BindTheData();
        }
    }
}

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About the Author

dave.dolan

United States United States
Dave works all day, and stays up all night coding and reading, surfing the intertubes.

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