Using trace back to get the size of a specific directory.

public long getDirSize(System.IO.DirectoryInfo dir)
{
	long size = 0;
	System.IO.DirectoryInfo[] dirs = dir.GetDirectories();
	System.IO.FileInfo[] files = dir.GetFiles();

	foreach (System.IO.FileInfo fi in files)
		size += fi.Length;

	foreach (System.IO.DirectoryInfo di in dirs)
		size += getDirSize(di);
	return size;
}

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