[DllImport("user32.dll")]
static extern IntPtr SetWindowsHookEx(int idHook, KeyboardProc callback, IntPtr hInstance, int threadId);

[DllImport("user32.dll")]
static extern bool UnhookWindowsHookEx(IntPtr hInstance);

[DllImport("user32.dll")]
static extern IntPtr CallNextHookEx(IntPtr idHook, int nCode, int wParam, IntPtr lParam);

[DllImport("kernel32.dll", CallingConvention = CallingConvention.StdCall)]
private static extern int GetCurrentThreadId();

[DllImport("kernel32.dll")]
static extern IntPtr LoadLibrary(string lpFileName);

private const int WH_KEYBOARD = 2;

public void SetHook()
{
    IntPtr hInstance = LoadLibrary("User32");
    hHook = SetWindowsHookEx(WH_KEYBOARD, _proc, hInstance, GetCurrentThreadId());
}

private const int HC_ACTION = 0;
private const int KF_REPEAT = 0x4000;

private IntPtr HookProc(int code, IntPtr wParam, IntPtr lParam)
{
    if (code == HC_ACTION)
    {
        var repeat = HiWord(lParam) & KF_REPEAT;
        
        if (repeat == 0)
        {
            Debug.WriteLine($"{code} / {wParam} / {lParam}");

            switch ((int)wParam)
            {
                case VK_F1:
                    this.txb.Text += "F1 was pressed" + Environment.NewLine; break;
                case VK_F2:
                    this.txb.Text += "F2 was pressed" + Environment.NewLine; break;
                case VK_F3:
                    this.txb.Text += "F3 was pressed" + Environment.NewLine; break;
                case VK_F4:
                    this.txb.Text += "F4 was pressed" + Environment.NewLine; break;
                case VK_F5:
                    this.txb.Text += "F5 was pressed" + Environment.NewLine; break;
                case VK_F6:
                    this.txb.Text += "F6 was pressed" + Environment.NewLine; break;
            }
        }
    }
    return CallNextHookEx(hHook, code, (int)wParam, lParam);
}

private static ulong HiWord(IntPtr ptr)
{
    if (((ulong)ptr & 0x80000000) == 0x80000000)
        return ((ulong)ptr >> 16);
    else
        return ((ulong)ptr >> 16) & 0xffff;
}