Dictionary that tells which key was not present, C#

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//Extended dictionary

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
    {
        public TValue this[TKey key]
        {
            get
            {
                TValue val;
                if(base.TryGetValue(key, out val))
                {
                    return val;
                }
                else
                {
                    throw new KeyNotFoundException(string.Format("The given key ({0}) was not present in the dictionary.", key));
                }
            }
            set
            {
                base[key] = value;
            }
        }
    }
    public class Program
    {
        public static void Main(string[] args)
        {
            //Dictionary<string, string> dic = new Dictionary<string, string>() 
            //{
            //    {"a", "b"}
            //};
            MyDictionary<string, string> dic = new MyDictionary<string, string>() 
            {
                {"a", "b"}
            };
            
            Console.WriteLine(dic["aa"]);
        }
    }
}

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Compilation time: 0.14 sec, absolute running time: 0.08 sec, cpu time: 0.06 sec, average memory usage: 16 Mb, average nr of threads: 4 (cached)