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Using Linq to filter collection with group By and select max

Posted on 2009-07-14
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Last Modified: 2013-11-11
Given the sample code shown below, how can I use linq to filter the collection to group By field1 and select the highest value of field2, so that my new collection would have classB and classC only in it?
public class Class1
    {
        public int field1;
        public int field2;
        public DateTime field3;
        public int field4;
    }
 
    public class Class1Collection : Collection<Class1>, IEnumerable<Class1>
    {
    }
 
 
    public static class TestClass
    {
 
        public static void Test()
        {
            Class1 classA = new Class1();
            classA.field1 = 1;
            classA.field2 = 2;
            classA.field3 = new DateTime(2009, 01, 01);
            classA.field4 = 3;
 
            Class1 classB = new Class1();
            classB.field1 = 1;
            classB.field2 = 3;
            classB.field3 = new DateTime(2009, 02, 01);
            classB.field4 = 3;
 
            Class1 classC = new Class1();
            classC.field1 = 2;
            classC.field2 = 1;
            classC.field3 = new DateTime(2009, 02, 01);
            classC.field4 = 3;
 
            Class1Collection collection = new Class1Collection();
            collection.Add(classA);
            collection.Add(classB);
            collection.Add(classC);
 
            Class1Collection filteredCollection = ??
        }
 
    }

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Question by:weimha
3 Comments
 
LVL 21

Assisted Solution

by:Craig Wagner
Craig Wagner earned 50 total points
ID: 24851703
Try this LINQ query...
from c in collection
group c by c.field1 into grouped
from g in grouped
where g.field2 == grouped.Max( m => m.field2 )
select g;

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LVL 62

Assisted Solution

by:Fernando Soto
Fernando Soto earned 50 total points
ID: 24851753
Hi weimha;

Here is some sample code showing how this can be done.

Fernando
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace WindowsFormsApplication45
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        List<Class1> collection = new List<Class1>();
 
        private void button1_Click(object sender, EventArgs e)
        {
            Class1 classA = new Class1();
            classA.field1 = 1;
            classA.field2 = 2;
            classA.field3 = new DateTime(2009, 01, 01);
            classA.field4 = 3;
 
            Class1 classB = new Class1();
            classB.field1 = 1;
            classB.field2 = 3;
            classB.field3 = new DateTime(2009, 02, 01);
            classB.field4 = 3;
 
            Class1 classC = new Class1();
            classC.field1 = 2;
            classC.field2 = 1;
            classC.field3 = new DateTime(2009, 02, 01);
            classC.field4 = 3;
 
            collection.Add(classA);
            collection.Add(classB);
            collection.Add(classC);
 
            List<Class1> filteredCollection = (from c in collection
                                               group c by c.field1 into classGroup
                                               let max = classGroup.Max(m => m.field2)
                                               from m in classGroup
                                               where m.field2 == max
                                               select m).ToList();
 
            // Display the results
            foreach (Class1 c1 in filteredCollection)
            {
                Console.WriteLine("{0,4}{1,4}{2,24}{3,4}",
                    c1.field1, c1.field2, c1.field3, c1.field4);
            }
 
        }
    }
 
    public class Class1
    {
        public int field1;
        public int field2;
        public DateTime field3;
        public int field4;
    }
}

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LVL 15

Accepted Solution

by:
jinal earned 400 total points
ID: 24851810

public class Class1
    {
        public int field1;
        public int field2;
        public DateTime field3;
        public int field4;
    }
 
    public class Class1Collection : Collection<Class1>, IEnumerable<Class1> , IList<Class1>
    {
    }
 
 
    public static class TestClass
    {
 
        public static void Test()
        {
            Class1 classA = new Class1();
            classA.field1 = 1;
            classA.field2 = 2;
            classA.field3 = new DateTime(2009, 01, 01);
            classA.field4 = 3;
 
            Class1 classB = new Class1();
            classB.field1 = 1;
            classB.field2 = 3;
            classB.field3 = new DateTime(2009, 02, 01);
            classB.field4 = 3;
 
            Class1 classC = new Class1();
            classC.field1 = 2;
            classC.field2 = 1;
            classC.field3 = new DateTime(2009, 02, 01);
            classC.field4 = 3;
 
            Class1Collection collection = new Class1Collection();
            collection.Add(classA);
            collection.Add(classB);
            collection.Add(classC);
 
 
            IEnumerable<Class1> data =  from cc in collection 
                        join cc1 in (
                        from c in collection
                        group c by c.field1 into g
                        select new { ID = g.Key, Max = g.Max(p => p.field2) }) on cc.field1 equals cc1.ID  where cc.field2 == cc1.Max select cc;
 
 
            Class1Collection filtercollection = new Class1Collection();
            foreach (Class1 c in data)
            {
                filtercollection.Add(c);
            }
            
            
            
 
        }
 
    }

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