Capturing keystrokes with GetAsyncKeyState

November 5, 2003 on 3:09 pm | In General |

An easy way to capture keystrokes in an application even if it is not the active one is by using the API function, GetAsyncKeyState, of the User32 DLL.

The trick is to set up a timer that regularly checks for the state of certain keys with GetAsyncKeyState

Here is an example:

using System;
using System.Windows.Forms;

public class Test {

	protected Timer timer = null;

	public Test {
		timer = new Timer();
		timer.Interval = 100; // check the keystate every 100ms
		timer.Click += new EventHandler(OnTimerTick);
		timer.Start();
	}

	private void OnTimerTick(Object sender, EventArgs args) {
		bool hotKeyPressed = true;
		// check the status of the left Windows key and the
		// right Control key
		Keys[] keys = {Keys.LWin, Keys.RControlKey};
		for (int i = 0; i < keys.Length; i++) {
			hotKeyPressed &= (GetAsyncKeyState( (int) keys[i]) & 0x8000) == 0x8000;
		}
		if (hotKeyPressed) {
			// do something
		}
	}

	// we expose the API function in the following lines
	[DllImport("User32.Dll")]
	private static extern int GetAsyncKeyState(int keyCode);

}

This technique is used in “TiBar”:http://www.jeysoft.com to detect whether the application launcher is being called.

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