How to create predicate dynamically

Question

Hi i want to create a list based on the search string using predicate expressions.

I have a list of type products contains different names.

List<products> list1 = new List<products>();

        list1.Add(new products("sowmya"));
        list1.Add(new products("Jane"));
        list1.Add(new products("John"));
        list1.Add(new products("kumar"));
        list1.Add(new products("ramya"));
        listBox1.ItemsSource = list1;

Now i want to filter the content based on user input.User will enter n no of strings with '+' as separator. After receiving the strings i will pass them to predicate object like this

 private void textBox1_KeyDown(object sender, KeyEventArgs e)
    {
        List<products> list2 = new List<products>();
        Expression<Func<products, bool>> predicate = PredicateBuilder.True<products>();
        if (e.Key == Key.Enter)
        {
            string Searchstring = textBox1.Text.ToString().Trim();
            string[] separator = new string[] { "+" };
            string[] SearchItems=Searchstring.Split(separator,StringSplitOptions.None);
            foreach (string str in SearchItems)
            {
                string temp = str;
                predicate =p => p.Name.Contains(temp.ToLower());                   
            }

            list2 = list1.AsQueryable().Where(predicate).ToList();
            listBox1.ItemsSource = list2;
        }
    }

If i enter more than one string(sowmya+jane+john) its giving only the last string(john) result but i want a list of all matching strings

Please answer this question because i'm trying this but i couldn't get the result.

Please do some help thanks.

Ironsmith

dup: #845559 – Sightread

Answer 1

Initialize the predicate as false

Expression<Func<products, bool>> predicate = PredicateBuilder.False<products>();

You need to combine the predicates using Or

foreach (string str in SearchItems)
{
    string temp = str;
    predicate = predicate.Or(p => p.NameToLower().Contains(temp.ToLower()));                   
}

Source for predicate builder here. It is part of LINQKit

Code, in case link goes

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

public static class PredicateBuilder
{
  public static Expression<Func<T, bool>> True<T> ()  { return f => true;  }
  public static Expression<Func<T, bool>> False<T> () { return f => false; }

  public static Expression<Func<T, bool>> Or<T> (this Expression<Func<T, bool>> expr1,
                                                      Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters);
  }

  public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
                                                       Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters);
  }
}

Seymourseys

If i use And to combine predicates its giving all the list items but not the matching string result – Ironsmith

@Ironsmith updated my answer. you need to use temp not str inside the predicate – Seymourseys

Hey Eranga thank you soooo much its working fine. Really you helped me – Ironsmith

Is this predicatebuilder the same to this article albahari.com/nutshell/predicatebuilder.aspx? – Chuddar

PredicateBuilder? – Oblast

Answer 2

As I am not sure that predicate instance has an And method, I suggest you use this code:

var list = list1.AsQueryable();
foreach (string str in SearchItems)
{
     list = list.Where(p => p.Name.ToLower().Contains(str.ToLower()));
}
listBox1.ItemsSource = list.ToList();

Cabochon

if i use this code its giving only one string result but if i enter two strings its not giving any result.Please find the solution.I need it. – Ironsmith

that might be because of the first .ToLower() I just added. if not make sure your Name property really contains both of the strings you search for – Cabochon

same issue again giving the last string result only.Ya i'm sure The "Name" property contains both the strings – Ironsmith

you should check if you have copied and pasted the code correctly then. – Cabochon

Answer 3

You don't have to build a predicate here. You can try something like this

List<products> list1 = new List<products>();

list1.Add(new products("sowmya"));
list1.Add(new products("Jane"));
list1.Add(new products("John"));
list1.Add(new products("kumar"));
list1.Add(new products("ramya"));

string input = "aaa+kuma+ram";
List<string> searchStrings =
    input.Split(new string[] { "+" }, StringSplitOptions.None)
    .Select(s => s.ToLower())
    .ToList();

List<products> list2 = (
    from p in list1
    where searchStrings.Any(s => p.Name.Contains(s))
    select p).ToList();

list2 will contain "kumar" and "ramya".

Hamadryad

Thank u and this will work and working also i knew that but i need to construct it by using predicate only please help me – Ironsmith

@Ironsmith then you should try what Eranga suggests. – Hamadryad

Sorry, but this is just the classical way of using predicates in LINQ. There's nothing dynamic here. – Cohleen

The OP had a goal, he thought he needed dynamic predicates to reach that goal. I showed how to reach it without dynamic predicates. – Hamadryad