C# Get Folder Size

Add a reference to the Microsoft Scripting Runtime and use:

string path = @"d:\folderpath";
Scripting.FileSystemObject fso = new Scripting.FileSystemObject();
Scripting.Folder folder = fso.GetFolder(path);
long dirSize = (long)folder.Size;

Referencing this COM seems to be one of the fastest ways to get a folder size. I reached faster times with this than with recursive (SearchOption.AllDirectories) System.IO.DirectoryInfo > EnumerateFiles.

A quick test performed repeatedly on 30K files (6.5GB) internal 7200rpm 3.5 SATA:

EnumerateFiles: ~580 ms.
FilesSystemObject: ~170ms.