How to close application in Xamarin forms cross-platform

Can i have the code to close the whole application in Xamarin Forms Cross-Platform PCL.
I written the below code in button click

protected void BtnExit_Click(object sender, EventArgs args)
{
if (Device.OS == TargetPlatform.Android)
{

        }
        if (Device.OS == TargetPlatform.iOS) 
        {

        }
        if (Device.OS == TargetPlatform.WinPhone)
        {

        }
    }

Thanks in advance