C# Keys Enumeration Confused: Keys.Alt or Keys.RButton | Keys.ShiftKey | Keys.Alt

Question

I was trying to test whether the Alt key was pressed.

I had a check similar to:

private void ProcessCmdKey(Keys keyData)
{
 if (keyData == Keys.Alt)
 {
  System.Console.WriteLine ("Alt Key Pressed");
 } 
}

Anyways needless to say when I breakpointed when I had pressed the Alt key the debugger told me the key that was pressed was actually Keys.RButton | Keys.ShiftKey | Keys.Alt

Can anyone shed some light on what is going on or perhaps point me to an article that can explain?

Thanks FZ

Edit: I am still a bit lost as to why the ENUM would have have other bit values set and not simply the Alt key? I understand that the enum can include more than 1 state with the flags attrivbute but I am not sure why it does if all I pressed was Alt?

Answer 1

If you want to test whether Alt is part of the pressed keys, you can perform a bitwise test;

if((keyData & Keys.Alt) == Keys.Alt) {...}

Answer 2

Keys is a Flags Enumeration. This means it can have more than one value at a given time. You should check it like so:

if ( (keyData & Keys.Alt) == Keys.Alt)
{
   // Alt was pressed!
}

Answer 3

Enum with FlagsAttribute are implemented using bits.
See this link for a good start - http://msdn.microsoft.com/en-us/library/cc138362.aspx

EDIT: Are you pressing RIGHT (mouse button) with Shift key during the operation, to select/highlight something, while debugging?

Answer 4

Mark's technique (the accepted answer) works for modifier keys, but it caught me by surprise that some keys (e.g. arrows) are combinations of bits, and won't work. For example the following test turns out to be true:

((Keys.Right & Keys.Left) == Keys.Left) 

I have posted some useful little functions for key handling on a related StackOverflow post regarding arrow key handling.