using System;
using System.Linq;

public class Test
{
	public static void Main()
	{
var items = new[] {
    new { id =1, Name = "item-1"},
    new { id =2, Name = "item-2"},
    new { id =3, Name = "item-3"},
    new { id =4, Name = "item-4"}
};
var issues = new[] {
    new { id =1, Date = "01.01.2017"},
    new { id =2, Date = "01.02.2017"}
};
var joined = from item in items
             join issue in issues on item.id equals issue.id into gj
             from sub in gj.DefaultIfEmpty()
             select new { item.id, item.Name, isPlanned=sub?.Date != null, sub?.Date };
foreach (var t in joined) {
    Console.WriteLine("{0} {1} {2} {3}", t.id, t.Name, t.isPlanned, t.Date);
}

	}
}