Detect Shift key is pressed without using events in Windows Forms?

Question

I need to be able to detect that the shift key is being held, but I don\'t want to use events or global variables to determine that. Is there an API in C# that lets you ask

Answer 1

You can install a low-level keyboard hook.

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

class InterceptKeys
{
    private const int WH_KEYBOARD_LL = 13;
    private const int WM_KEYDOWN = 0x0100;
    private static LowLevelKeyboardProc _proc = HookCallback;
    private static IntPtr _hookID = IntPtr.Zero;

    public static void Main()
    {
        _hookID = SetHook(_proc);
        Application.Run();
        UnhookWindowsHookEx(_hookID);
    }

    private static IntPtr SetHook(LowLevelKeyboardProc proc)
    {
        using (Process curProcess = Process.GetCurrentProcess())
        using (ProcessModule curModule = curProcess.MainModule)
        {
            return SetWindowsHookEx(WH_KEYBOARD_LL, proc,
                GetModuleHandle(curModule.ModuleName), 0);
        }
    }

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

    private static IntPtr HookCallback(
        int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
        {
            int vkCode = Marshal.ReadInt32(lParam);
            Console.WriteLine((Keys)vkCode);
        }
        return CallNextHookEx(_hookID, nCode, wParam, lParam);
    }

    [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);
}

Answer 2

if ((Control.ModifierKeys & Keys.Shift) != 0) 

This will also be true if another modifier key is also down (eg, Ctrl+Shift). If you want to check whether Shift alone is pressed without any other modifiers, use

if (Control.ModifierKeys == Keys.Shift)

Note that even this will be true if another non-modifier is down (Eg, Shift+A). If you want to check whether Shift and only Shift is pressed, you'll have to use an API call.

---

If you're in a class that inherits Control (such as a form), you can remove the Control qualifier. (static properties don't need qualifiers in inherited classes)

Answer 3

Form.ModifierKeys (static property)

Answer 4

Not sure if this available in C# but you can call GetAsyncKeyState. This method returns the state of a key at the time the method is called.

To call it from C# you'll need to use interop like any other Win32 API.