How to use the SendInput method correctly?

[vitkuz573]

I'm trying to use the SendInput method and don't quite understand how to do it right. At the moment my code looks something like this (example):

if (deltaY < 0)
{
    deltaY = -120;
}
else if (deltaY > 0)
{
    deltaY = 120;
}

var union = new INPUT._Anonymous_e__Union
{
    mi = new MOUSEINPUT
    {
        dwFlags = MOUSE_EVENT_FLAGS.MOUSEEVENTF_WHEEL,
        dx = 0,
        dy = 0,
        time = 0,
        mouseData = deltaY,
        dwExtraInfo = 0
    }
};

var input = new INPUT
{
    type = INPUT_TYPE.INPUT_MOUSE,
    Anonymous = union
};

var inputs = new Span<INPUT>();
inputs.Fill(input);

SendInput(inputs, Marshal.SizeOf(typeof(INPUT)));

But it doesn't work. That's why I have this question.

[AArnott]

Ultimately I think your error is that you create a span of zero length, and then try to 'fill' it, but nothing will happen because it's empty.

-var inputs = new Span<INPUT>();
-inputs.Fill(input);
+var inputs = new Span<INPUT>(ref input);

But that relies on a new constructor for .NET 7. If you're not that lucky, this will also work:

Span<INPUT> inputs = stackalloc INPUT[1];
inputs[0] = input;

I would also touch up a bit of the other syntax, leaving me with:

var input = new INPUT
{
    type = INPUT_TYPE.INPUT_MOUSE,
    Anonymous =
    {
        mi = new MOUSEINPUT
        {
            dwFlags = MOUSE_EVENT_FLAGS.MOUSEEVENTF_WHEEL,
            dx = 0,
            dy = 0,
            time = 0,
            mouseData = deltaY switch { < 0 => -120, > 0 => 120, 0 => 0 },
            dwExtraInfo = 0
        },
    },
};

#if NET7_0_OR_GREATER
Span<INPUT> inputs = new(ref input);
#else
Span<INPUT> inputs = stackalloc INPUT[1];
inputs[0] = input;
#endif

PInvoke.SendInput(inputs, Marshal.SizeOf(typeof(INPUT)));

I haven't tested this, but I'm optimistic it'll work. :)

[vitkuz573]

Sorry, in another use case everything works. Apparently I'm missing something, I'll look into it. Thanks for the reply!

[Zintom]

How do you supply a negative value to mouseData?, it is an unsigned integer.

[vitkuz573]

It worked for me

mouseData = data.DeltaY < 0 ? 120u : data.DeltaY > 0 ? unchecked((uint)-120) : 0u

[AArnott]

@vitkuz573's workaround is correct, and unfortunately this isn't something we can look forward to being improved in the metadata. microsoft/win32metadata#1865 (comment)

[Zintom]

Thanks, I resolved it by casting the result of the switch expression as a whole which worked too.