Handle key event when the app is running in background.

GlobalInput.cs

using System;
using System.Runtime.InteropServices;
using System.Diagnostics;

#region WIN32 TYPES
using DWORD = System.UInt64;
using HINSTANCE = System.IntPtr;
using HHOOK = System.IntPtr;
using WPARAM = System.IntPtr;
using LPARAM = System.IntPtr;
using LRESULT = System.IntPtr;
using HMODULE = System.IntPtr;
using LPCWSTR = System.String;
using ULONG_PTR = System.UIntPtr;
#endregion

namespace GlobalInput
{
    public enum HookType : int
    {
        WH_JOURNALRECORD = 0,
        WH_JOURNALPLAYBACK = 1,
        WH_KEYBOARD = 2,
        WH_GETMESSAGE = 3,
        WH_CALLWNDPROC = 4,
        WH_CBT = 5,
        WH_SYSMSGFILTER = 6,
        WH_MOUSE = 7,
        WH_HARDWARE = 8,
        WH_DEBUG = 9,
        WH_SHELL = 10,
        WH_FOREGROUNDIDLE = 11,
        WH_CALLWNDPROCRET = 12,
        WH_KEYBOARD_LL = 13,
        WH_MOUSE_LL = 14
    }

    public enum GlobalKey
    {
        LeftButton = 0x01,
        RightButton = 0x02,
        Cancel = 0x03,
        MiddleButton = 0x04,
        ExtraButton1 = 0x05,
        ExtraButton2 = 0x06,
        Back = 0x08,
        Tab = 0x09,
        Clear = 0x0C,
        Return = 0x0D,
        Shift = 0x10,
        Control = 0x11,
        Menu = 0x12,
        Pause = 0x13,
        CapsLock = 0x14,
        Kana = 0x15,
        Hangeul = 0x15,
        Hangul = 0x15,
        Junja = 0x17,
        Final = 0x18,
        Hanja = 0x19,
        Kanji = 0x19,
        Escape = 0x1B,
        Convert = 0x1C,
        NonConvert = 0x1D,
        Accept = 0x1E,
        ModeChange = 0x1F,
        Space = 0x20,
        Prior = 0x21,
        Next = 0x22,
        End = 0x23,
        Home = 0x24,
        Left = 0x25,
        Up = 0x26,
        Right = 0x27,
        Down = 0x28,
        Select = 0x29,
        Print = 0x2A,
        Execute = 0x2B,
        Snapshot = 0x2C,
        Insert = 0x2D,
        Delete = 0x2E,
        Help = 0x2F,
        N0 = 0x30,
        N1 = 0x31,
        N2 = 0x32,
        N3 = 0x33,
        N4 = 0x34,
        N5 = 0x35,
        N6 = 0x36,
        N7 = 0x37,
        N8 = 0x38,
        N9 = 0x39,
        A = 0x41,
        B = 0x42,
        C = 0x43,
        D = 0x44,
        E = 0x45,
        F = 0x46,
        G = 0x47,
        H = 0x48,
        I = 0x49,
        J = 0x4A,
        K = 0x4B,
        L = 0x4C,
        M = 0x4D,
        N = 0x4E,
        O = 0x4F,
        P = 0x50,
        Q = 0x51,
        R = 0x52,
        S = 0x53,
        T = 0x54,
        U = 0x55,
        V = 0x56,
        W = 0x57,
        X = 0x58,
        Y = 0x59,
        Z = 0x5A,
        LeftWindows = 0x5B,
        RightWindows = 0x5C,
        Application = 0x5D,
        Sleep = 0x5F,
        Numpad0 = 0x60,
        Numpad1 = 0x61,
        Numpad2 = 0x62,
        Numpad3 = 0x63,
        Numpad4 = 0x64,
        Numpad5 = 0x65,
        Numpad6 = 0x66,
        Numpad7 = 0x67,
        Numpad8 = 0x68,
        Numpad9 = 0x69,
        Multiply = 0x6A,
        Add = 0x6B,
        Separator = 0x6C,
        Subtract = 0x6D,
        Decimal = 0x6E,
        Divide = 0x6F,
        F1 = 0x70,
        F2 = 0x71,
        F3 = 0x72,
        F4 = 0x73,
        F5 = 0x74,
        F6 = 0x75,
        F7 = 0x76,
        F8 = 0x77,
        F9 = 0x78,
        F10 = 0x79,
        F11 = 0x7A,
        F12 = 0x7B,
        F13 = 0x7C,
        F14 = 0x7D,
        F15 = 0x7E,
        F16 = 0x7F,
        F17 = 0x80,
        F18 = 0x81,
        F19 = 0x82,
        F20 = 0x83,
        F21 = 0x84,
        F22 = 0x85,
        F23 = 0x86,
        F24 = 0x87,
        NumLock = 0x90,
        ScrollLock = 0x91,
        NEC_Equal = 0x92,
        Fujitsu_Jisho = 0x92,
        Fujitsu_Masshou = 0x93,
        Fujitsu_Touroku = 0x94,
        Fujitsu_Loya = 0x95,
        Fujitsu_Roya = 0x96,
        LeftShift = 0xA0,
        RightShift = 0xA1,
        LeftControl = 0xA2,
        RightControl = 0xA3,
        LeftMenu = 0xA4,
        RightMenu = 0xA5,
        BrowserBack = 0xA6,
        BrowserForward = 0xA7,
        BrowserRefresh = 0xA8,
        BrowserStop = 0xA9,
        BrowserSearch = 0xAA,
        BrowserFavorites = 0xAB,
        BrowserHome = 0xAC,
        VolumeMute = 0xAD,
        VolumeDown = 0xAE,
        VolumeUp = 0xAF,
        MediaNextTrack = 0xB0,
        MediaPrevTrack = 0xB1,
        MediaStop = 0xB2,
        MediaPlayPause = 0xB3,
        LaunchMail = 0xB4,
        LaunchMediaSelect = 0xB5,
        LaunchApplication1 = 0xB6,
        LaunchApplication2 = 0xB7,
        OEM1 = 0xBA,
        OEMPlus = 0xBB,
        OEMComma = 0xBC,
        OEMMinus = 0xBD,
        OEMPeriod = 0xBE,
        OEM2 = 0xBF,
        OEM3 = 0xC0,
        OEM4 = 0xDB,
        OEM5 = 0xDC,
        OEM6 = 0xDD,
        OEM7 = 0xDE,
        OEM8 = 0xDF,
        OEMAX = 0xE1,
        OEM102 = 0xE2,
        ICOHelp = 0xE3,
        ICO00 = 0xE4,
        ProcessKey = 0xE5,
        ICOClear = 0xE6,
        Packet = 0xE7,
        OEMReset = 0xE9,
        OEMJump = 0xEA,
        OEMPA1 = 0xEB,
        OEMPA2 = 0xEC,
        OEMPA3 = 0xED,
        OEMWSCtrl = 0xEE,
        OEMCUSel = 0xEF,
        OEMATTN = 0xF0,
        OEMFinish = 0xF1,
        OEMCopy = 0xF2,
        OEMAuto = 0xF3,
        OEMENLW = 0xF4,
        OEMBackTab = 0xF5,
        ATTN = 0xF6,
        CRSel = 0xF7,
        EXSel = 0xF8,
        EREOF = 0xF9,
        Play = 0xFA,
        Zoom = 0xFB,
        Noname = 0xFC,
        PA1 = 0xFD,
        OEMClear = 0xFE
    }

    public enum MessageType: int
    {
        WM_KEYDOWN = 0x0100, 
        WM_KEYUP = 0x0101, 
        WM_SYSKEYDOWN = 0x0104, 
        WM_SYSKEYUP = 0x0105
    }

    public delegate LRESULT HOOKPROC(int nCode, WPARAM wParam, LPARAM lParam);

    [StructLayout(LayoutKind.Sequential)]
    public struct KBDLLHOOKSTRUCT
    {
        public DWORD vkCode;
        public DWORD scanCode;
        public DWORD flags;
        public DWORD time;
        public ULONG_PTR dwExtraInfo;
    }

    public abstract class GlobalInput
    {
        public static class NativeMethods
        {
            [DllImport("user32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
            internal static extern HHOOK SetWindowsHookExW(
                int idHook, HOOKPROC lpfn, HINSTANCE hMod, DWORD dwThreadId);

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

            [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
            internal static extern LRESULT CallNextHookEx(
                HHOOK hhk, int nCode, WPARAM wParam, LPARAM lParam);

            [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
            internal static extern HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
        }

        private HOOKPROC hook_proc;
        private HHOOK hook_handle = IntPtr.Zero;

        public string SetHook()
        {
            if(hook_handle == IntPtr.Zero)
            {
                hook_proc = HookProc;
                using (var proccess = Process.GetCurrentProcess())
                {
                    using (var module = proccess.MainModule)
                    {
                        HMODULE module_handle = NativeMethods.GetModuleHandleW(module.ModuleName);
                        hook_handle = NativeMethods.SetWindowsHookExW(
                            (int)HookType.WH_KEYBOARD_LL, hook_proc, module_handle, 0);
                    }
                }
            }

            if(hook_handle == IntPtr.Zero)
            {
                int errno = Marshal.GetLastWin32Error();
                return $"Failed to set hook. (ErrNo: ${errno})";
            }

            return "";
        }

        public string UnsetHook()
        {
            if(!NativeMethods.UnhookWindowsHookEx(hook_handle))
            {
                int errno = Marshal.GetLastWin32Error();
                return $"Failed to unset hook. (ErrNo: ${errno})";
            }
            hook_handle = IntPtr.Zero;

            return "";
        }

        protected virtual LRESULT HookProc(int nCode, WPARAM wParam, LPARAM lParam)
        {
            return NativeMethods.CallNextHookEx(hook_handle, nCode, wParam, lParam);
        }
    }

    public class GlobalKeyInput: GlobalInput
    {

        public static GlobalKeyInput Instance = new GlobalKeyInput();
        private GlobalKeyInput() { }

        public static string Enable()
        {
            return Instance.SetHook();
        }

        public static string Disable()
        {
            return Instance.UnsetHook();
        }

        public static event Action<GlobalKey> KeyDownEvent
        {
            add { Instance._KeyDownEvent += value; }
            remove { Instance._KeyDownEvent -= value;  }
        }

        public static event Action<GlobalKey> KeyUpEvent
        {
            add { Instance._KeyUpEvent += value; }
            remove { Instance._KeyUpEvent -= value; }
        }

        private event Action<GlobalKey> _KeyDownEvent;
        private event Action<GlobalKey> _KeyUpEvent;

        protected void OnKeyDown(GlobalKey key)
        {
            _KeyDownEvent?.Invoke(key);
        }

        protected void OnKeyUp(GlobalKey key)
        {
            _KeyUpEvent?.Invoke(key);
        }

        protected override LRESULT HookProc(int nCode, WPARAM wParam, LPARAM lParam)
        {
            if(nCode >= 0)
            {
                var kb = Marshal.PtrToStructure<KBDLLHOOKSTRUCT>(lParam);
                var vkCode = (GlobalKey)kb.vkCode;

                if (wParam == (IntPtr)MessageType.WM_KEYDOWN 
                    || wParam == (IntPtr)MessageType.WM_SYSKEYDOWN)
                {
                    OnKeyDown(vkCode);
                }
                else if (wParam == (IntPtr)MessageType.WM_KEYUP
                    || wParam == (IntPtr)MessageType.WM_SYSKEYUP)
                {
                    OnKeyUp(vkCode);
                }
            }
            return base.HookProc(nCode, wParam, lParam);
        }
    }
}