C# Signature:

[DllImport("kernel32.dll", SetLastError=true, CharSet=CharSet.Auto)]
[return: MarshalAs(UnmanagedType.Bool)]
   static extern bool GetDiskFreeSpaceEx(string lpDirectoryName,
   out ulong lpFreeBytesAvailable,
   out ulong lpTotalNumberOfBytes,
   out ulong lpTotalNumberOfFreeBytes);

User-Defined Types:

None.

static extern bool GetDiskFreeSpace(string lpRootPathName,
   out uint lpSectorsPerCluster,
   out uint lpBytesPerSector,
   out uint lpNumberOfFreeClusters,
   out uint lpTotalNumberOfClusters);

Notes:

None.

VB.Net Signature:

Declare Auto Function GetDiskFreeSpace Lib "kernel32.dll" ( _
     ByVal lpRootPathName As String, _
     ByRef lpSectorsPerCluster As UInt32, _
     ByRef lpBytesPerSector As UInt32, _
     ByRef lpNumberOfFreeClusters As UInt32, _
     ByRef lpTotalNumberOfClusters As UInt32) As Integer

Tips & Tricks:

None.

Notes:

In the .Net Framework, this functionality is available in the System.IO.DriveInfo class.

Sample Code:

ulong FreeBytesAvailable;
ulong TotalNumberOfBytes;
ulong TotalNumberOfFreeBytes;

On Windows 95/98 the GetDiskFreeSpace function cannot report volume sizes that are greater than 2 gigabytes (GB). To ensure that your application works with large capacity hard drives, use the GetDiskFreeSpaceEx function. On Windows NT and Windows 2000, GetDiskFreeSpace is not limited to 2GB; it reports the full size of the drive and the full amount of free space. See http://support.microsoft.com/kb/231497 .

bool success = GetDiskFreeSpaceEx("C:\\", out FreeBytesAvailable, out TotalNumberOfBytes,
                   out TotalNumberOfFreeBytes);
if (!success)
    throw new System.ComponentModel.Win32Exception();

Console.WriteLine("Free Bytes Available:      {0,15:D}", FreeBytesAvailable);
Console.WriteLine("Total Number Of Bytes:     {0,15:D}", TotalNumberOfBytes);
Console.WriteLine("Total Number Of FreeBytes: {0,15:D}", TotalNumberOfFreeBytes);

Tips & Tricks:

If you are reading information for multiple drives and/or do not need sector cluster information, use the Managed API instead (System.IO.DriveInfo)

Sample Code:

uint SectorsPerCluster;
uint BytesPerSector;
uint NumberOfFreeClusters;
uint TotalNumberOfClusters;

GetDiskFreeSpace("C:\\", out SectorsPerCluster, out BytesPerSector,
   out NumberOfFreeClusters, out TotalNumberOfClusters);

Console.WriteLine("Sectors Per Cluster: {0}", SectorsPerCluster);
Console.WriteLine("Bytes Per Sector: {0}", BytesPerSector);
Console.WriteLine("Number Of Free Clusters: {0}", NumberOfFreeClusters);
Console.WriteLine("Total Number Of Clusters: {0}", TotalNumberOfClusters);    

// SectorsPerCluster * BytesPerSection will give you how many bytes are available per sector
// And by multiplying the result with NumberOfFreeClusters gives you the free space in bytes.
long Bytes = (long)NumberOfFreeClusters * SectorsPerCluster * BytesPerSector;
Console.WriteLine("Total Free Space in bytes: {0} ", Bytes);

decimal KiloBytes = (decimal)Bytes / 1024;
Console.WriteLine("Total Free Space in kilo bytes: {0}", KiloBytes);

decimal MegaBytes = (decimal)KiloBytes / 1024;
Console.WriteLine("Total Free Space in mega bytes: {0}", MegaBytes);

decimal GigaBytes = (decimal)MegaBytes / 1024;
Console.WriteLine("Total Free Space in giga bytes: {0}", GigaBytes);

Console.WriteLine("Please enter any key to exit...");
Console.ReadLine();

Alternative Managed API:

Starting in v2.0 (Whidbey) of the .NET Framework, this is exposed through the System.Io.DriveInfo class.

Unfortunately DriveInfo is not working for UNC Paths, for them you still have to use the above function

Windows CE:

The ManagedWindowsApi project (http://mwinapi.sourceforge.net) provides an ExtendedFileInfo class that can get sector/cluster sizes and counts.

The call is also available via coredll.dll on CE devices:

[DllImport("coredll.dll", SetLastError=true, CharSet=CharSet.Auto)]
[return: MarshalAs(UnmanagedType.Bool)]
   static extern bool GetDiskFreeSpaceEx(string lpDirectoryName,
   out ulong lpFreeBytesAvailable,
   out ulong lpTotalNumberOfBytes,
   out ulong lpTotalNumberOfFreeBytes);

Here is a C# wrapper for the call

/// <summary>
/// Determines the ammount of free space on a drive (local or remote) in bytes
/// </summary>
/// <param name="folderName">Directory or unc folder name of the volume to
/// be checked (can be a networked drive)</param>
/// <param name="freespace">Ammount of freespace is returned here in bytes, 0
/// if the value could not be obtained</param>
/// <returns>true if the method successfully retrieved the ammount of freespace
/// on the given drive, false  otherwise</returns>
public static bool DriveFreeBytes(string folderName, out ulong freespace)
{
   freespace = 0;
   if (string.IsNullOrEmpty(folderName))
   {
     throw new ArgumentNullException("folderName");
   }

   if (!folderName.EndsWith("\\"))
   {
     folderName += '\\';
   }

   ulong free = 0, dummy1 = 0, dummy2 = 0;

   if (GetDiskFreeSpaceEx(folderName, out free, out dummy1, out dummy2))
   {
     freespace = free;
     return true;
   }
   else
   {
     return false;
   }
}

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