using System;

using System.Collections.Generic; 

​

//###################################################################################################

//https://metadataconsulting.blogspot.com/2019/11/C-Sharp-Convert-a-GUID-to-a-Compressed-GUID.html

//###################################################################################################

public class Program

{

    public static bool IsHex(IEnumerable<char> chars)

    {

        bool isHex; 

        foreach(var c in chars)

        {

            isHex = ((c >= '0' && c <= '9') || 

                     (c >= 'a' && c <= 'f') || 

                     (c >= 'A' && c <= 'F'));

​

            if(!isHex)

                return false;

        }

        return true;

    }

    public static void Main()

    {

           //Source - http://www.hanselman.com/blog/BATCHFILEVOODOODetermineIfMultipleAndWhichVersionsOfAnMSIinstalledProductAreInstalledUsingUpgradeCode.aspx

           //like to but to bulky - https://docs.microsoft.com/en-us/dotnet/standard/base-types/how-to-define-and-use-custom-numeric-format-providers

           //https://installpac.wordpress.com/2008/03/31/packed-guids-darwin-descriptors-and-windows-installer-reference-counting/

           //PROTO    

           //{12345678-ABCD-WXYZ-1234-ABCDEFGHIJKL}

           //should be-> 87654321DCBAZYXW2134BADCFEHGJILK

           //string inStrGUID = "{12345678-ABCD-WXYZ-1234-ABCDEFGHIJKL}"; //cannot use this contrived GUID has hex values above > (A-F) 

           string inStrGUID = "{12345678-ABCD-EFAB-1234-ABCDEFABCDEF}"; 

           string expectedCompressedGUID = "87654321DCBABAFE2143BADCFEBADCFE";

           //https://docs.microsoft.com/en-us/dotnet/api/system.guid.parse?view=netframework-4.7.2 - N has no dashes

           string outputFormat = "N"; //outputFormat can be N, D, B, P - N has no dashes

           string outCompressedGuid = ""; 

           string outDecompressedGuid = ""; 

           Guid inGuid = new Guid();

           Guid outGuid = new Guid(); 

           Guid outDCGuid = new Guid(); 

           bool isValidGuid = Guid.TryParse(inStrGUID, out inGuid); 

           Console.WriteLine("Input GUID " + inStrGUID+ " is valid? "+isValidGuid); 

           if (isValidGuid) {

               string raw = inGuid.ToString("N"); //N for no dashes

               char[] aRaw = raw.ToCharArray();

               //compressed format reverses 11 byte sequences of the original guid

               int[] revs

                   = new int[]{8, 4, 4, 2, 2, 2, 2, 2, 2, 2, 2};

               int pos = 0;

               for (int i = 0; i < revs.Length; i++)

               {

                   Array.Reverse(aRaw, pos, revs[i]);

                   pos += revs[i];

               }

               string newstrGuid = new string(aRaw);

               bool isoutValidGuid = Guid.TryParse(newstrGuid, out outGuid); 

               if ( isoutValidGuid ) {

               //GUID in registry are all caps.

               outCompressedGuid = outGuid.ToString(outputFormat).ToUpper(); 

                Console.WriteLine("out  CGUID "+   outGuid.ToString("B").ToUpper()+" before full compression (removal of dashes) here for readability"); //for readability

                Console.WriteLine("\nCompressed guid is "+ outCompressedGuid );

                if (outCompressedGuid == expectedCompressedGUID) 

                    Console.WriteLine("Matched expectations."); 

               else

                   Console.WriteLine("Did not met expectations.");   

               } else

                   Console.WriteLine("Failed to compress GUID."); 

           }

           Console.WriteLine("\nInput Compressed GUID " + outCompressedGuid); 

            char[] chrArrCGUID = outCompressedGuid.ToCharArray();

           //Here's a typical GUID Compressed in a Registry Key                

           //what if ? S-1-5-21-xxxxxxxxxx-xxxxxxxxx-xxxxxxxxx-xxx - split on new char[] {' ','-'} check any that are length 32 

           //Expand/Decompress/Decrypt Compressed GUID

           if (outCompressedGuid.Length == 32 && Program.IsHex(chrArrCGUID)==true){ //validate potential Compressed GUID

               int[] reversalidxs= new int[]{8, 4, 4, 2, 2, 2, 2, 2, 2, 2, 2};

               int pos = 0;

               for (int i = 0; i < reversalidxs.Length; i++)

               {

                   Array.Reverse(chrArrCGUID, pos, reversalidxs[i]);

                   pos += reversalidxs[i];

               }

               string newstrGuid = new string(chrArrCGUID);

               bool isoutValidDCGuid = Guid.TryParse(newstrGuid, out outDCGuid );

               if ( isoutValidDCGuid ) {

                   //GUID in registry are all caps.

                   outDecompressedGuid = outDCGuid.ToString("B").ToUpper(); 

​

                   //Console.WriteLine("\nInput Compressed GUID "+   outDCGuid.ToString("B").ToUpper()+" this line for comparison only"); //for readability

                   Console.WriteLine("Output Decomprsd GUID "+   outDCGuid.ToString("N").ToUpper()+" this line for comparison only"); //for readability

​

                   Console.WriteLine("\nDecompressed compressed guid is "+ outDecompressedGuid );

                 }

           }

           else 

              Console.WriteLine("Failed to decompressed CGUID.");  

    }

}