C# Online Compiler | .NET Fiddle

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.");  
		
	}
	
}

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

Cached Result

Input GUID {12345678-ABCD-EFAB-1234-ABCDEFABCDEF} is valid? True
out CGUID {87654321-DCBA-BAFE-2143-BADCFEBADCFE} before full compression (removal of dashes) here for readability

Compressed guid is 87654321DCBABAFE2143BADCFEBADCFE
Matched expectations.

Input Compressed GUID 87654321DCBABAFE2143BADCFEBADCFE
Output Decomprsd GUID 12345678ABCDEFAB1234ABCDEFABCDEF this line for comparison only

Decompressed compressed guid is {12345678-ABCD-EFAB-1234-ABCDEFABCDEF}