Why does GetAsyncKeyState registers CTRL when pressing ALT

Question

I am currently using GetAsyncKeyState to register key presses in a csharp program.

The issue I'm running into is that when pressing right Alt (RMenu / vKey 165), the program also registers a right Control press (RControlKey / vKey 162).

I'm running a for loop to check every vKey integer of the method like this :

for (int i = 0; i < 254; i++)
{
    short keyState = GetAsyncKeyState(i);
    if(keyState < 0)
    {
        Debug.WriteLine(i);
    }
}

I tried filtering the RControl key out, however I'd like to also detect this one.

Am I missing something and is there a way to work around it ?

Answer 1

I think the issue is how you are checking if the key is pressed.

According to the documentation, you should compare the most significant bit. Try this instead:

Import the function in this manner:

[DllImport("User32.dll")]
public static extern short GetAsyncKeyState(Keys ArrowKeys);

Then you use a binary AND operator to check if the most significant digit is set on the value returned by the imported function:

for (int i = 0; i < 254; i++)
{
    var key = (Keys)i;
    bool isKeyDown = (GetAsyncKeyState(key) & 0x8000) > 0; 
    if (isKeyDown)
    {
        Console.WriteLine($"{i} => {key}");
    }
}

The output, when the right alt key is pressed:

18 => Menu 
165 => RMenu 

You see both because from the list of keys, 18 indicates "The ALT key" and 165 is a more specific value that indicates "The right ALT key."

Similarly, pressing the right control key produces this output:

17 => ControlKey
163 => RControlKey