How to change the System Volume programmatically

Question

Hai Every one ...I want to change the System volume .I am using fallowing code......
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;
namespace shaped_form
{
class VolumeChange
{
private const int APPCOMMAND_VOLUME_MUTE = 0x80000;
private const int APPCOMMAND_VOLUME_UP = 0xA0000;
private const int APPCOMMAND_VOLUME_DOWN = 0x90000;
private const int WM_APPCOMMAND = 0x319;
[DllImport("user32.dll")]
public static extern IntPtr SendMessageW(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);
private void btnMute_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle,(IntPtr)APPCOMMAND_VOLUME_MUTE);
}
private void btnDecVol_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle,(IntPtr)APPCOMMAND_VOLUME_DOWN);
}
private void btnIncVol_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND,this.Handle,(IntPtr)APPCOMMAND_VOLUME_UP);
}
}
}
But I got an error  at the highlighted places...as shown below...i got this code from some internet sources