不在焦点时响应键盘？(C#,Vista)

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

public class GlobalKeyboardHook
{
    private const int WH_KEYBOARD_LL = 13;
    private const int WM_KEYDOWN = 0x0100;
    private const int WM_SYSKEYDOWN = 0x0104;

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr SetWindowsHookEx(int idHook, LowLevelKeyboardProc lpfn, IntPtr hMod, uint dwThreadId);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool UnhookWindowsHookEx(IntPtr hhk);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr GetModuleHandle(string lpModuleName);

    public delegate IntPtr LowLevelKeyboardProc(int nCode, IntPtr wParam, IntPtr lParam);

    public event EventHandler<KeyEventArgs> KeyDown;

    private IntPtr _hookId = IntPtr.Zero;
    private LowLevelKeyboardProc _proc;

    public GlobalKeyboardHook()
    {
        _proc = HookCallback;
    }

    public void Hook()
    {
        if (_hookId != IntPtr.Zero) return;

        using (Process curProcess = Process.GetCurrentProcess())
        using (ProcessModule curModule = curProcess.MainModule)
        {
            _hookId = SetWindowsHookEx(WH_KEYBOARD_LL, _proc, GetModuleHandle(curModule.ModuleName), 0);
        }
    }

    public void Unhook()
    {
        if (_hookId == IntPtr.Zero) return;

        UnhookWindowsHookEx(_hookId);
        _hookId = IntPtr.Zero;
    }

    private IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode >= 0)
        {
            int vkCode = Marshal.ReadInt32(lParam);
            if (wParam == (IntPtr)WM_KEYDOWN || wParam == (IntPtr)WM_SYSKEYDOWN)
            {
                KeyDown?.Invoke(this, new KeyEventArgs((Keys)vkCode));
            }
        }

        return CallNextHookEx(_hookId, nCode, wParam, lParam);
    }
}

public class KeyEventArgs : EventArgs
{
    public Keys KeyCode { get; }

    public KeyEventArgs(Keys keyCode)
    {
        KeyCode = keyCode;
    }
}