using System;
using System.Runtime.InteropServices;
using System.Threading;
using System.Windows.Forms;

class Program {

	[DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
	static extern IntPtr GetProcAddress(IntPtr hModule, string procName);

	[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Ansi)]
	static extern IntPtr LoadLibrary([MarshalAs(UnmanagedType.LPStr)]string lpFileName);

	// Function to be hooked
	[DllImport("kernel32.dll")]
	static extern void Sleep(uint dwMilliseconds);

	// Must create delegate type
	// Marshal.GetFunctionPointerForDelegate requires non generic delegate tpe
	delegate void SleepFx(uint ms);

	static FxHook hook;
	// Hook function must be of same signature as of original function
	static void Sleep2(uint ms) {
		MessageBox.Show("Sleeping for: " + ms.ToString());
		// Uninstall hook before calling real function otherwise stack overflow
		hook.Uninstall();
		Sleep(ms);
		hook.Install();
	}
	static void Main(string[] args) {
		var k32 = LoadLibrary("kernel32");
		var slp = GetProcAddress(k32, "Sleep");
		using(hook = new FxHook(slp, (SleepFx)Sleep2)) {
			hook.Install();
			Sleep(1000);
		}
		// This will call original sleep
		Sleep(1000);
	}
	

}