Performance – using Guid object or Guid string as Key

The Guid should be quicker, as the comparison is simpler – just a few direct bytes. The string involves a dereference and lots more work.

Of course – you could profile ;-p

Evidence:

Searching for 7f9b349f-f36f-94de-ad96-04279ddf6ecf
As guid: 466; -1018643328
As string: 512; -1018643328
Searching for 870ba465-08f2-c872-cfc9-b3cc1ffa09de
As guid: 470; 1047183104
As string: 589; 1047183104
Searching for d2376f8a-b8c9-4633-ee8e-9679bb30f918
As guid: 423; 1841649088
As string: 493; 1841649088
Searching for 599889e8-d5fd-3618-4c4f-cb620e6f81bb
As guid: 488; -589561792
As string: 493; -589561792
Searching for fb64821e-c541-45f4-0fd6-1c772189dadf
As guid: 450; 1389733504
As string: 511; 1389733504
Searching for 798b9fe5-ba15-2753-357a-7637161ee48a
As guid: 415; 779298176
As string: 504; 779298176
Searching for 12ba292e-8e59-e5d0-7d04-e811a237dc21
As guid: 457; 558250944
As string: 564; 558250944
Searching for 05b3ce14-dfbf-4d3a-1503-ced515decb81
As guid: 413; 1658205056
As string: 504; 1658205056
Searching for 8db4a556-0a65-d8cb-4d0d-0104245d18b8
As guid: 415; 696231936
As string: 506; 696231936
Searching for c49cf80c-5537-fba5-eebd-8ad21bba09c4
As guid: 459; 2100976384
As string: 557; 2100976384

based on:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
static class Program
{

    static void Main()
    {
        Random rand = new Random(123456);
        int COUNT = 1000;
        Dictionary<Guid, int> guids = new Dictionary<Guid, int>(COUNT);
        Dictionary<string, int> strings = new Dictionary<string, int>(
            COUNT, StringComparer.Ordinal);

        byte[] buffer = new byte[16];
        for (int i = 0; i < COUNT; i++)
        {
            rand.NextBytes(buffer);
            Guid guid = new Guid(buffer);
            int val = rand.Next();
            guids.Add(guid, val);
            strings.Add(guid.ToString(), val);
        }

        for(int i = 0 ; i < 10 ; i++) {
            int index = rand.Next(COUNT);
            Guid guid = guids.Keys.Skip(index).First();
            Console.WriteLine("Searching for " + guid);
            int chk = 0;
            const int LOOP = 5000000;
            Stopwatch watch = Stopwatch.StartNew();
            for (int j = 0; j < LOOP; j++)
            {
                chk += guids[guid];
            }
            watch.Stop();
            Console.WriteLine("As guid: " + watch.ElapsedMilliseconds
                   + "; " + chk);
            string key = guid.ToString();
            chk = 0;
            watch = Stopwatch.StartNew();
            for (int j = 0; j < LOOP; j++)
            {
                chk += strings[key];
            }
            watch.Stop();
            Console.WriteLine("As string: " + watch.ElapsedMilliseconds
                   + "; " + chk);
        }
        Console.ReadLine();

    }
}