Question

I need to fire an event when the mouse is above a PictureBox with the mouse button already clicked and held down.

Problems:

The MouseDown and MouseEnter event handlers do not work together very well.

For instance once a mouse button is clicked and held down, C# will fire the MouseDown event handler, but when the cursor moves over the PictureBox the MouseEnter event does not fire, until the mouse button is realeased.

Answer 1

When the mouse is pressed down most controls will then Control.Capture the mouse input. This means that all MouseMove events are sent to the original control that captured rather than the control the mouse happens to be over. This continues until the mouse loses capture which typically happens on the mouse up.

If you really need to know when the mouse is over your control even when another control has captured mouse input then you only really have one way. You need to snoop the windows messages destined for other controls inside your application. To do that you need add a message filter …

Application.AddMessageFilter(myFilterClassInstance); 

Then you need to implement the IMessageFilter on a suitable class…

public class MyFilterClass : IMessageFilter {     public bool PreFilterMessage(ref Message m)     {         if (m.Msg == WM_MOUSEMOVE)             // Check if mouse is over my picture box!          return false;     } } 

Then you watch for mouse move events and check if they are over your picture box and do whatever it is you want to do.