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