using System;
using Microsoft.Win32;

namespace GetAssociatedApp
{
    class Program
    {
        static void Main(string[] args)
        {
            const string extPathTemplate = @"HKEY_CLASSES_ROOT\{0}";
            const string cmdPathTemplate = @"HKEY_CLASSES_ROOT\{0}\shell\open\command";

            // 1. Find out document type name for .jpeg files
            const string ext = ".jpeg";

            var extPath = string.Format(extPathTemplate,ext);

            var docName = Registry.GetValue(extPath,string.Empty,string.Empty) as string;
            if (!string.IsNullOrEmpty(docName))
            {
                // 2. Find out which command is associated with our extension
                var associatedCmdPath = string.Format(cmdPathTemplate,docName);
                var associatedCmd = 
                    Registry.GetValue(associatedCmdPath,string.Empty) as string;

                if (!string.IsNullOrEmpty(associatedCmd))
                {
                    Console.WriteLine("\"{0}\" command is associated with {1} extension",associatedCmd,ext);
                }
            }
        }
    }
}