using System;
using System.Linq;
using System.Collections.Generic;
 
public class P
{
	struct DataItem
	{
		public System.DateTime time;
		public int number;
	}
 
	public static void Main(string[] args)
	{
		var ItemList = new DataItem[] {} ;
		var groups = ItemList
			.GroupBy(item => new { item.time.Hour, item.time.Minute });
		var sums   = groups
			.ToDictionary(g => g.Key, g => g.Sum(item => item.number));
 
 
		// lookups now become trivially easy:
 
		int partialByTimeSlot = sums[new {Hour=23,Minute=10}];
	}
}
 

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