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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace WindowsFormsApplication5
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            var x = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
            var y = new string[] { "A", "B", "C", "D", "E", "F", "G", "H", "I"};
            var data = new[] {
                new {X = 0, Y = "A", Value = "0A"},
                new {X = 2, Y = "B", Value = "2B"},
                new {X = 5, Y = "F", Value = "5F"},
                new {X = 8, Y = "A", Value = "8A"}
            };
 
            var result = from a in x
                         from b in y
                         join c in data.DefaultIfEmpty() on a + b equals c.X + c.Y into d
                         let f = d.FirstOrDefault()
                         select new { X = a, Y = b , Value = (f == null ? "" : f.Value) };
 
            MessageBox.Show(string.Join("\r\n", result));
        }
    }
}
 

prog.cs(4,14): error CS0234: The type or namespace name `Data' does not exist in the namespace `System'. Are you missing an assembly reference?
prog.cs(8,24): error CS0234: The type or namespace name `Tasks' does not exist in the namespace `System.Threading'. Are you missing an assembly reference?
prog.cs(9,14): error CS0234: The type or namespace name `Windows' does not exist in the namespace `System'. Are you missing an assembly reference?
Compilation failed: 3 error(s), 0 warnings

Standard output is empty