Automating Key Presses with SendInput

Question

When I attempt to use SendInput to send single key presses, and combined keypresses, I can't get my program to hold the keyboard button until commanded to be released. With the code below I am ables to send the character 'a', and 'A', by hitting shift first. However, I cannot get it to hold the 'a' button in perpetuity.

public static void KeyDown()
{
    SwitchWindow(Process.GetProcessesByName("notepad").FirstOrDefault().MainWindowHandle);
    INPUT[] inputs = new INPUT[1];
    KEYBDINPUT kb = new KEYBDINPUT();

    //Set up generic keyboard event
    inputs[0].type = INPUT_KEYBOARD;
    kb.wScan = 0; // hardware scan code for key
    kb.time = 0;
    kb.dwExtraInfo = IntPtr.Zero;

    kb.dwFlags = 0; // 0 for key press

    //Press shift
    kb.wVk = (ushort)KeyCode.SHIFT;
    inputs[0].ki = kb;
    SendInput(1, inputs, Marshal.SizeOf(inputs[0]));

    //Press 'a' key
    kb.wVk = (ushort)0x41; // virtual-key code for the "a" key
    inputs[0].ki = kb;
    SendInput(1, inputs, Marshal.SizeOf(inputs[0]));

    //Release 'a' key
    kb.dwFlags = KEYEVENTF_KEYUP;
    kb.wVk = (ushort)0x41; // virtual-key code for the "a" key
    inputs[0].ki = kb;
    SendInput(1, inputs, Marshal.SizeOf(inputs[0]));

    //Release 'shift' key
    kb.dwFlags = KEYEVENTF_KEYUP;
    kb.wVk = (ushort)KeyCode.SHIFT; // virtual-key code for the "a" key
    inputs[0].ki = kb;
    SendInput(1, inputs, Marshal.SizeOf(inputs[0]));
}

Any idea why, if I remove the last two SendInputs, it doesn't just hold 'A' down?

Because "perpetually holding down a button" causes continual input to a program, you need to simulate the repeated key press messages. — Peter Ritchie, Apr 21 '14 at 20:21
To mimic holding the 'a' down manually, you would need to send multiple key-down events just the keyboard auto-repeat mechanism does. — user1793036, Apr 22 '14 at 01:25
Thanks guys. Two quick follow-up questions: Why do I need the release button code if it just presses once anyways? The code above works, for holding shift and pressing 'a' for instance (outputs 'A'). Why is it that for shift it actually holds it down, but for 'a' it seems to just press it once? Thanks again. — Magnus, Apr 22 '14 at 06:51

score 2 · Accepted Answer · answered Apr 30 '14 at 14:46

For the benefit of others that are searching for how to use SendInput() in C#, please see the below description.

1) There are two ways to send the key stroke with SendInput(), either using a Scancode or a virtual-key code

2) If you intend to send keystrokes to a game or another application that takes input from Direct 3D (or another DirectInput environment), you need to send the key strokes as scancodes, and therefor you specify the wScan field, and leave wVk blank. See info here: KEYBDINPUT

3) Remember to switch the window in focus to the application you want to send keystrokes to, before sending the keystroke. Example:

MemoryHandler.SwitchWindow(Process.GetProcessesByName("notepad").FirstOrDefault().MainWindowHandle);

4) Final code:

    public static void PressKey(char ch, bool press)
    {
        byte vk = MemoryApi.VkKeyScan(ch);
        PressKey((MemoryApi.KeyCode)vk, press);
    }

    public static void PressKey(MemoryApi.KeyCode vk, bool press)
    {
        ushort scanCode = (ushort)MemoryApi.MapVirtualKey((ushort)vk, 0);

        //Console.WriteLine("SendInput:: VK: " + (ushort)vk + " (" + vk + ") <-> SC: " + (ushort)(scanCode & 0xff));

        if (press)
            KeyDown(scanCode);
        else
            KeyUp(scanCode);
    }

    public static void KeyDown(ushort scanCode)
    {
        //Console.WriteLine("Key Down (SC): " + (ushort)(scanCode & 0xff));
        MemoryApi.INPUT[] inputs = new MemoryApi.INPUT[1];

        inputs[0].type = MemoryApi.INPUT_KEYBOARD;
        inputs[0].ki.wScan = (ushort)(scanCode & 0xff);
        inputs[0].ki.dwFlags = MemoryApi.KEYEVENTF_SCANCODE;
        inputs[0].ki.time = 0;
        inputs[0].ki.dwExtraInfo = IntPtr.Zero;

        uint intReturn = MemoryApi.SendInput(1, inputs, Marshal.SizeOf(inputs[0]));
        if (intReturn != 1)
        {
            throw new Exception("Could not send key: " + scanCode);
        }
    }

    public static void KeyUp(ushort scanCode)
    {
        //Console.WriteLine("Key Up (SC): " + scanCode);
        MemoryApi.INPUT[] inputs = new MemoryApi.INPUT[1];

        inputs[0].type = MemoryApi.INPUT_KEYBOARD;
        inputs[0].ki.wScan = (ushort)(scanCode & 0xff);
        inputs[0].ki.dwFlags = MemoryApi.KEYEVENTF_SCANCODE | MemoryApi.KEYEVENTF_KEYUP;
        inputs[0].ki.time = 0;
        inputs[0].ki.dwExtraInfo = IntPtr.Zero;

        uint intReturn = MemoryApi.SendInput(1, inputs, Marshal.SizeOf(inputs[0]));
        if (intReturn != 1)
        {
            throw new Exception("Could not send key: " + scanCode);
        }
}

What is this MemoryApi that you are referring to? – Mort Oct 26 '17 at 09:22 — Mort, Oct 26 '17 at 09:22

Automating Key Presses with SendInput

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