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()
    {