File Path/size in C#

using System.IO;
string directoryName = Path.GetDirectoryName(@"C:myfoldersubfoldermydoc.pdf");
// directoryName now contains "C:myfoldersubfolder"

public static long DirSize(DirectoryInfo d)
{
    long Size = 0;
    // Add file sizes.
    FileInfo[] fis = d.GetFiles();
    foreach (FileInfo fi in fis)
    {
        Size += fi.Length;
    }
    // Add subdirectory sizes.
    DirectoryInfo[] dis = d.GetDirectories();
    foreach (DirectoryInfo di in dis)
    {
        Size += DirSize(di);
    }
    return(Size);
}