它使用ho1建议的GetCompressedFileSize，以及PaulStack建议的GetDiskFreeSpace，但是使用P /
Invoke。我仅对压缩文件进行了测试，我怀疑它对零碎文件不起作用。

    public static long GetFileSizeOnDisk(string file)
    {
        FileInfo info = new FileInfo(file);
        uint dummy, sectorsPerCluster, bytesPerSector;
        int result = GetDiskFreeSpaceW(info.Directory.Root.FullName, out sectorsPerCluster, out bytesPerSector, out dummy, out dummy);
        if (result == 0) throw new Win32Exception();
        uint clusterSize = sectorsPerCluster * bytesPerSector;
        uint hosize;
        uint losize = GetCompressedFileSizeW(file, out hosize);
        long size;
        size = (long)hosize << 32 | losize;
        return ((size + clusterSize - 1) / clusterSize) * clusterSize;
    }

    [DllImport("kernel32.dll")]
    static extern uint GetCompressedFileSizeW([In, MarshalAs(UnmanagedType.LPWStr)] string lpFileName,
       [Out, MarshalAs(UnmanagedType.U4)] out uint lpFileSizeHigh);

    [DllImport("kernel32.dll", SetLastError = true, PreserveSig = true)]
    static extern int GetDiskFreeSpaceW([In, MarshalAs(UnmanagedType.LPWStr)] string lpRootPathName,
       out uint lpSectorsPerCluster, out uint lpBytesPerSector, out uint lpNumberOfFreeClusters,
       out uint lpTotalNumberOfClusters);