Question

How do I select the top of the list as long as the items at the top of the list have a property with a specific value.

I need the Linq statement to see that there is a break in the sequence and the only return the top two items. The problem is that I don’t know exactly how many items are going to have the correct property value.

I have been working through this problem with LinqPad 4. The code below is a copy and past from LinqPad 4. The results, “q”, should not contain SomeData with an EffectiveDate of 4/5/2011 because the Kind property on hsc2 is “KindTwo”.

I am trying to find the value most resent value of “Kind” and then only take the top records that match that value until I get to a record that doesn’t match that value.

void Main()
{
    var hsc1 = new SomeData {EffectiveDate = new DateTime(2011,4,5), Kind = "KindOne"};
    var hsc2 = new SomeData {EffectiveDate = new DateTime(2011,4,10), Kind = "KindTwo"};
    var hsc3 = new SomeData {EffectiveDate = new DateTime(2011,4,20), Kind = "KindOne"};
    var hsc4 = new SomeData {EffectiveDate = new DateTime(2011,4,25), Kind = "KindOne"};

    var all = new [] {hsc1, hsc2, hsc3, hsc4};

    var lastSomeData = all.OrderByDescending((x) => x.EffectiveDate).First();

    lastSomeData.Dump();

    var q = from h in all 
            where h.Kind == lastSomeData.Kind
            orderby h.EffectiveDate descending 
            select h;

    q.Dump();
}

// Define other methods and classes here
class SomeData
{
    public DateTime EffectiveDate {get;set;}
    public string Kind {get;set;}
}

Answer 1

This is a fully working Console Application that does what you asked. As I was not the first to propose the use of TakeWhile in this question, please do not mark my answer as the accepted one.

using System;
using System.Linq;
namespace stackoverflow.com_questions_5825629_select_topx_while_x_kind_value
{
    class Program
    {
        static void Main()
        {
            var hsc1 = new SomeData { EffectiveDate = new DateTime(2011, 4, 5), Kind = "KindOne" };
            var hsc2 = new SomeData { EffectiveDate = new DateTime(2011, 4, 10), Kind = "KindTwo" };
            var hsc3 = new SomeData { EffectiveDate = new DateTime(2011, 4, 20), Kind = "KindOne" };
            var hsc4 = new SomeData { EffectiveDate = new DateTime(2011, 4, 25), Kind = "KindOne" };

            var all = new[] { hsc1, hsc2, hsc3, hsc4 };

            var lastSomeData = all.OrderByDescending((x) => x.EffectiveDate).First();

            var q = (from h in all
                     orderby h.EffectiveDate descending
                     select h).TakeWhile(x => x.Kind == lastSomeData.Kind);

            var result = q.ToArray();

            foreach (var item in result)
                Console.WriteLine(item);
            Console.WriteLine("");
            Console.WriteLine("Press any key");
            Console.ReadKey();
        }

        // Define other methods and classes here
        class SomeData
        {
            public DateTime EffectiveDate { get; set; }
            public string Kind { get; set; }
            public override string ToString()
            {
                return string.Format(@"new SomeData {{ EffectiveDate = new DateTime({0}, {1}, {2}), Kind = ""{3}"" }};", EffectiveDate.Year, EffectiveDate.Month, EffectiveDate.Day, Kind);
            }
        }
    }
}