static void Main(string[] args)
        {
            string dir = @"C:\";
            string[] dirs=Directory.GetDirectories(dir);
            long totalSize = 0;
            if(dirs!=null && dirs.Any())
            {
                foreach(string dr in dirs)
                {
                    var size = new DirectoryInfo(dr).GetDirectorySize();
                    totalSize += size;
                    Console.WriteLine($"dir:{dr},size:{size}");
                }
            }
            Console.WriteLine($"totalSize:{totalSize}");
            System.Diagnostics.Debug.WriteLine($"totalSize:{totalSize}");
        }  

static class DirHelper
    {
        public static long GetDirectorySize(this System.IO.DirectoryInfo directoryInfo, bool recursive = true)
        {
            var startDirectorySize = default(long);
            try
            {
                
                if (directoryInfo == null || !directoryInfo.Exists)
                    return startDirectorySize; //Return 0 while Directory does not exist.

                //Add size of files in the Current Directory to main size.
                foreach (var fileInfo in directoryInfo.GetFiles())
                    System.Threading.Interlocked.Add(ref startDirectorySize, fileInfo.Length);

                if (recursive) //Loop on Sub Direcotries in the Current Directory and Calculate it‘s files size.
                    System.Threading.Tasks.Parallel.ForEach(directoryInfo.GetDirectories(), (subDirectory) =>
                System.Threading.Interlocked.Add(ref startDirectorySize, GetDirectorySize(subDirectory, recursive)));

                  //Return full Size of this Directory.
            }
            catch
            {

            }
            return startDirectorySize;
        }
    }