using System;

using System.Runtime.InteropServices;

​

public class Program

{

    public static void Main()

    {

        ulong available  = 0;

        ulong totalBytes = 0;

        ulong totalFree  = 0;

​

        string dir = @"C:\";

​

        GetDiskFreeSpaceEx1(dir, out available, out totalBytes, out totalFree);

        Console.WriteLine(available);

​

        GetDiskFreeSpaceEx2(dir, ref available, ref totalBytes, ref totalFree);

        Console.WriteLine(available);

    }

​

    [DllImport("kernel32.dll", EntryPoint = "GetDiskFreeSpaceEx", SetLastError = true, CharSet = CharSet.Auto)]

    [return: MarshalAs(UnmanagedType.Bool)]

    static extern bool GetDiskFreeSpaceEx1(

        [In]  string    lpDirectoryName,

        [Out] out ulong lpFreeBytesAvailable,

        [Out] out ulong lpTotalNumberOfBytes,

        [Out] out ulong lpTotalNumberOfFreeBytes);

​

    [DllImport("kernel32.dll", EntryPoint = "GetDiskFreeSpaceEx", SetLastError = true, CharSet = CharSet.Auto)]

    [return: MarshalAs(UnmanagedType.Bool)]

    static extern bool GetDiskFreeSpaceEx2(

        [In]  string    lpDirectoryName,

        [In, Out] ref ulong lpFreeBytesAvailable,

        [In, Out] ref ulong lpTotalNumberOfBytes,

        [In, Out] ref ulong lpTotalNumberOfFreeBytes);  

}

​