var size = new DirectoryInfo("E:\\").GetDirectorySize();
and here's the code behind this Extension method
public static long GetDirectorySize(this System.IO.DirectoryInfo directoryInfo, bool recursive = true)
{
var startDirectorySize = default(long);
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 startDirectorySize; //Return full Size of this Directory.
}