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Finding a Time of Interest in a Dictionary

Given a dictionary list whose keys are a list of dates, how would you find the item that has a date <= a specific time of interest.

For example: Given the following dates in the dictionary and a Time of Interest = 11am, the result should be 6am.

6am on 1/2/2009
12am on 1/2/2009
6pm on 1/2/2009

Here’s a little test I threw together to test this out. Can you improve on it?

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
 
 
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            // forecasts by date (populated earlier)
            DateTime mTimeResult;
            Dictionary<DateTime, string> dForecast = new Dictionary<DateTime, string>();
 
            dForecast.Add(new DateTime(2009, 2, 4, 6, 0, 0), "6 am");
            dForecast.Add(new DateTime(2009, 2, 4, 12, 0, 0), "12 Noon");
            dForecast.Add(new DateTime(2009, 2, 4, 18, 0, 0), "6 pm");
 

            // Test 1 - Time of Interest is less than the dates in the dictionary
            mTimeResult = FindInterest(dForecast, new DateTime(2008, 1, 4, 15, 0, 0));
            System.Diagnostics.Debug.Assert(mTimeResult == DateTime.MinValue, "BAD Test 1");
  
            // Test 2 - Time of Interest is set to the dates in the dictionary and set to 5am.
            mTimeResult = FindInterest(dForecast, new DateTime(2009, 2, 4, 5, 0, 0));
            System.Diagnostics.Debug.Assert(mTimeResult == DateTime.MinValue, "BAD Test 2");
  
            // Test 3 - Time of Interest is in the dictionary and set to 6am. 
            mTimeResult = FindInterest(dForecast, new DateTime(2009, 2, 4, 6, 0, 0));
            System.Diagnostics.Debug.Assert(mTimeResult == new DateTime(2009, 2, 4, 6, 0, 0), "BAD Test 3");
  
            // Test 4 - Time of Interest is in the dictionary and set to 11am. 
            mTimeResult = FindInterest(dForecast, new DateTime(2009, 2, 4, 11, 0, 0));
            System.Diagnostics.Debug.Assert(mTimeResult == new DateTime(2009, 2, 4, 6, 0, 0), "BAD Test 4");
  
            // Test 5 - Time of Interest is in the dictionary and set to 1pm. 
            mTimeResult = FindInterest(dForecast, new DateTime(2009, 2, 4, 13, 0, 0));
            System.Diagnostics.Debug.Assert(mTimeResult == new DateTime(2009, 2, 4, 12, 0, 0), "BAD Test 5");
  
            // Test 6 - Time of Interest is in the dictionary and set to 7pm. 
            mTimeResult = FindInterest(dForecast, new DateTime(2009, 2, 4, 19, 0, 0));
            System.Diagnostics.Debug.Assert(mTimeResult == DateTime.MinValue, "BAD Test 6");
 
            // Test 7 - Time of Interest is in the dictionary and set to DateTime.MaxValue. 
            mTimeResult = FindInterest(dForecast, DateTime.MaxValue);
            System.Diagnostics.Debug.Assert(mTimeResult == DateTime.MinValue, "BAD Test 7");
 
        }
 
        public static DateTime FindInterest(Dictionary<DateTime, string> dForecast, DateTime mTimeInterest)
        {
            // Find the key that corresponds to our needs or return DateTime.MinValue;
            return dForecast.Where(x => x.Key <= mTimeInterest).LastOrDefault().Key;
        }
    }
}

So did you notice the bug?

What if the dictionary list is not sorted? What happens then?
You should sort the dictionary before the WHERE clause.