SendInput won't send basic Unicode to some windows

Question

I have a problem with this project in C#: When using WinAPI SendInput function

    /// <summary>
    /// Sends Unicode (UTF16) string to foreground window.
    /// </summary>
    /// <param name="inputString">String to be sent to foreground window.</param>
    internal static void Send(string inputString)
    {
        if (inputString == string.Empty)
        { return; }

        char[] chars = inputString.ToCharArray();
        INPUT[] pInputs = new INPUT[chars.Length * 2];

        for (int i = 0; i < chars.Length; i++)
        {
            UInt16 unicode = chars[i];
            pInputs[i * 2] = new INPUT();
            pInputs[i * 2].type = INPUT_KEYBOARD;
            pInputs[i * 2].ki.wVk = 0;
            pInputs[i * 2].ki.wScan = unicode;
            pInputs[i * 2].ki.dwFlags = KEYEVENTF_UNICODE;
            pInputs[i * 2].ki.time = 0;
            pInputs[i * 2].ki.dwExtraInfo = SetMessageExtraInfo(IntPtr.Zero);

            pInputs[i * 2 + 1] = new INPUT();
            pInputs[i * 2 + 1].type = INPUT_KEYBOARD;
            pInputs[i * 2 + 1].ki.wVk = 0;
            pInputs[i * 2 + 1].ki.wScan = unicode;
            pInputs[i * 2 + 1].ki.dwFlags = KEYEVENTF_UNICODE | KEYEVENTF_KEYUP;
            pInputs[i * 2 + 1].ki.time = 0;
            pInputs[i * 2 + 1].ki.dwExtraInfo = SetMessageExtraInfo(IntPtr.Zero);
        }
        uint nSent = SendInput((uint)chars.Length * 2, pInputs, Marshal.SizeOf(typeof(INPUT)));
        if (nSent == 0)
        {
            Debug.WriteLine("SendInput error " + GetLastError().ToString()); // error 87 : "The parameter is incorrect."
        }
    }

In desktop applications like Notepad or VS the code works fine, but in others it doesn't work with the simple English alphabet and punctuation. A string like "íáó" with special characters works explicitly/universally, but a string like "My car" doesn't. Obviously, the utf-16 values for the chars in "My car" are low, less than 100; the values for á and í are 225 and 237, respectively. Seemingly a superficial difference. Anyone know of a way to get regular English letters to be sent as Unicode to arbitrary windows using SendInput?

Answer 1

This is not documentation on MSDN, but when using KEYEVENTF_UNICODE, if a given Unicode codepoint requires the use of UTF-16 surrogates, you need to send both surrogates without KEYEVENTF_KEYUP, then send both surrogates with KEYEVENTF_KEYUP. The code you showed is not accounting for UTF-16 surrogates at all, it is sending down/up input for each surrogate independently. The two surrogates need to be "down" at the same time, then "up" at the same time.

Try something more like this instead (based on a previous answer I posted for the same issue in C++):

/// <summary>
/// Sends Unicode (UTF16) string to foreground window.
/// </summary>
/// <param name="inputString">String to be sent to foreground window.</param>
internal static void Send(string inputString)
{
    if (string.IsNullOrEmpty(inputString))
    { return; }

    char[] chars = inputString.ToCharArray();
    int len = chars.Length;
    INPUT[] pInputs = new INPUT[len * 2];

    UInt32 ExtraInfo = GetMessageExtraInfo();

    int i = 0, idx = 0;
    while (i < len)
    {
        UInt16 ch = chars[i++];

        if ((ch < 0xD800) || (ch > 0xDFFF))
        {
            pInputs[idx] = new INPUT();
            pInputs[idx].type = INPUT_KEYBOARD;
            pInputs[idx].ki.wVk = 0;
            pInputs[idx].ki.wScan = ch;
            pInputs[idx].ki.dwFlags = KEYEVENTF_UNICODE;
            pInputs[idx].ki.time = 0;
            pInputs[idx].ki.dwExtraInfo = ExtraInfo;
            ++idx;

            pInputs[idx] = new INPUT();
            pInputs[idx].type = INPUT_KEYBOARD;
            pInputs[idx].ki.wVk = 0;
            pInputs[idx].ki.wScan = ch;
            pInputs[idx].ki.dwFlags = KEYEVENTF_UNICODE | KEYEVENTF_KEYUP;
            pInputs[idx].ki.time = 0;
            pInputs[idx].ki.dwExtraInfo = ExtraInfo;
            ++idx;
        }
        else
        {
            UInt16 ch2 = chars[i++];

            pInputs[idx] = new INPUT();
            pInputs[idx].type = INPUT_KEYBOARD;
            pInputs[idx].ki.wVk = 0;
            pInputs[idx].ki.wScan = ch;
            pInputs[idx].ki.dwFlags = KEYEVENTF_UNICODE;
            pInputs[idx].ki.time = 0;
            pInputs[idx].ki.dwExtraInfo = ExtraInfo;
            ++idx;

            pInputs[idx] = new INPUT();
            pInputs[idx].type = INPUT_KEYBOARD;
            pInputs[idx].ki.wVk = 0;
            pInputs[idx].ki.wScan = ch2;
            pInputs[idx].ki.dwFlags = KEYEVENTF_UNICODE;
            pInputs[idx].ki.time = 0;
            pInputs[idx].ki.dwExtraInfo = ExtraInfo;
            ++idx;

            pInputs[idx] = new INPUT();
            pInputs[idx].type = INPUT_KEYBOARD;
            pInputs[idx].ki.wVk = 0;
            pInputs[idx].ki.wScan = ch;
            pInputs[idx].ki.dwFlags = KEYEVENTF_UNICODE | KEYEVENTF_KEYUP;
            pInputs[idx].ki.time = 0;
            pInputs[idx].ki.dwExtraInfo = ExtraInfo;
            ++idx;

            pInputs[idx] = new INPUT();
            pInputs[idx].type = INPUT_KEYBOARD;
            pInputs[idx].ki.wVk = 0;
            pInputs[idx].ki.wScan = ch2;
            pInputs[idx].ki.dwFlags = KEYEVENTF_UNICODE | KEYEVENTF_KEYUP;
            pInputs[idx].ki.time = 0;
            pInputs[idx].ki.dwExtraInfo = ExtraInfo;
            ++idx;
        }
    }

    uint nSent = SendInput((uint)pInputs.Length, pInputs, Marshal.SizeOf(typeof(INPUT)));
    if (nSent == 0)
    {
        Debug.WriteLine("SendInput error " + GetLastError().ToString());
    }
}