Get size of file on disk

This uses GetCompressedFileSize, as ho1 suggested, as well as GetDiskFreeSpace, as PaulStack suggested, it does, however, use P/Invoke. I have tested it only for compressed files, and I suspect it does not work for fragmented files.

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);

The code above does not work properly on Windows Server 2008 or 2008 R2 or Windows 7 and Windows Vista based systems as cluster size is always zero (GetDiskFreeSpaceW and GetDiskFreeSpace return -1 even with UAC disabled.) Here is the modified code that works.

C#

public static long GetFileSizeOnDisk(string file)
{
    FileInfo info = new FileInfo(file);
    uint clusterSize;
    using(var searcher = new ManagementObjectSearcher("select BlockSize,NumberOfBlocks from Win32_Volume WHERE DriveLetter = '" + info.Directory.Root.FullName.TrimEnd('\\') + "'") {
        clusterSize = (uint)(((ManagementObject)(searcher.Get().First()))["BlockSize"]);
    }
    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);

VB.NET

  Private Function GetFileSizeOnDisk(file As String) As Decimal
        Dim info As New FileInfo(file)
        Dim blockSize As UInt64 = 0
        Dim clusterSize As UInteger
        Dim searcher As New ManagementObjectSearcher( _
          "select BlockSize,NumberOfBlocks from Win32_Volume WHERE DriveLetter = '" + _
          info.Directory.Root.FullName.TrimEnd("\") + _
          "'")

        For Each vi As ManagementObject In searcher.[Get]()
            blockSize = vi("BlockSize")
            Exit For
        Next
        searcher.Dispose()
        clusterSize = blockSize
        Dim hosize As UInteger
        Dim losize As UInteger = GetCompressedFileSizeW(file, hosize)
        Dim size As Long
        size = CLng(hosize) << 32 Or losize
        Dim bytes As Decimal = ((size + clusterSize - 1) / clusterSize) * clusterSize

        Return CDec(bytes) / 1024
    End Function

    <DllImport("kernel32.dll")> _
    Private Shared Function GetCompressedFileSizeW( _
        <[In](), MarshalAs(UnmanagedType.LPWStr)> lpFileName As String, _
        <Out(), MarshalAs(UnmanagedType.U4)> lpFileSizeHigh As UInteger) _
        As UInteger
    End Function

According to MSDN social forums:

The size on disk should be the sum of the size of the clusters that store the file:
long sizeondisk = clustersize * ((filelength + clustersize - 1) / clustersize);
You'll need to dip into P/Invoke to find the cluster size; GetDiskFreeSpace() returns it.

See How to get the size on disk of a file in C#.

But please note the point that this will not work in NTFS where compression is switched on.