MouseDown and Click conflict

I was thinking counting pixels if the form is actually dragged, but there has to be a better way.

Nope, that's exactly how you have to do it.

This isn't just a software limitation; it's very much a practical one as well. If you think through the problem from a user's perspective, you'll immediately see the problem as well as the solution. Ask yourself, what is the difference between a click and a drag?

Both of them start with the mouse button going down over the object, but one of them ends with the mouse button going back up over the object in the same position and the other one ends with the mouse button going back up in a completely different position.

Since time machines haven't been perfected yet, you have no way of knowing this in advance.

So yes, you need to maintain some kind of a distance threshold, and if the pointer moves outside of that distance threshold while it is down over the object, then you consider it a drag. Otherwise, you consider it a click.

That distance threshold should not be 0. The user should not be required to hold the mouse completely still in order to initiate a click. A lot of users are sub-par mousers. They are very likely to twitch slightly when trying to click. If the threshold is 0, they'll end up doing a lot of inadvertent dragging when they try to click.

Of course, you don't actually have to worry about any of this or compute the drag threshold yourself. Instead, use the Windows default values, obtainable by calling the GetSystemMetrics function and specifying either SM_CXDRAG or SM_CYDRAG. (These might be exposed somewhere by the WinForms framework, but I don't think so. It's just as easy to P/Invoke them yourself.)

const int SM_CXDRAG = 68;
const int SM_CYDRAG = 69;
[DllImport("user32.dll")]
static extern int GetSystemMetrics(int index);

Point GetDragThreshold()
{
    return new Point(GetSystemMetrics(SM_CXDRAG), GetSystemMetrics(SM_CYDRAG));
}

In the field of UX/UI, this sort of thing is called hysteresis or debouncing, by analogy to the use of these terms in physics and electronics.

Unfortunatelly, at the point of time when "button-is-pressed" you don't know yet if the desired action is just a click or a drag-drop. You will find it out it later.

For a click, the determinant is "no movement" and "button up".

For a drag, the determinant is "movement" and "button up".

Hence, to disambiguate those interactions, you have to track not only the buttons, but also the movement. You do not need to track the overall movement, only the movement between button-down and button-up is interesting.

Those events are therefore a good place to start/stop the Mouse.Capture mechanisms (to dynamically present drag adorners and drop location hints), or, in simplier form - to store the origin and target of movement vector and check if the distance is > D (even if movement occurred, there should be some safe minimal distance within which the DRAG is canceleed. The mouse is "jaggy" sometimes, and people would really don't like your app to start dragging when they double click at the end of fast mouse pointer movement :) )

I found this solution, although it is for a double-click and a mouse down events:

void pictureBox_MouseDown(object sender, MouseEventArgs e)
{
if (e.Button == MouseButtons.Left && e.Clicks ==1)
{
PictureBox pb = (PictureBox)sender;
DoDragDrop((ImageData)pb.Tag, DragDropEffects.Copy);
}
}

source: http://code.rawlinson.us/2007/04/c-dragdrop-and-doubleclick.html