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.

Notes:

None.

Tips & Tricks:

None.

Sample Code:

ulong FreeBytesAvailable;
ulong TotalNumberOfBytes;
ulong TotalNumberOfFreeBytes;

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);

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 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;
   }
}