Creating non-rectangular forms that can be moved by dragging anywhere in the background area

Question

I want to move my form window by left-clicking and holding on the background area, just like we normally do using the title bar.

And then I also want to change the form window of my application (i.e., a rectangular window) to my custom-designed graphical window. I have seen this type of window in many other applications, so I believe it is possible.

score 5 · Accepted Answer · edited May 23 '17 at 10:30

Question 1:

To allow a form to be moved when dragging its client area, you need to tell the window manager to treat the client area as if it were the title bar (caption area). You suggest something similar in your question.

This can be done in .NET by overriding the WndProc method of your form, responding to the WM_NCHITTEST message, and returning HTCAPTION to indicate that everything should be treated as part of the caption (title) bar, instead of the default HTCLIENT, which indicates that it should be treated as the form's client area. Add the following code to your form class:

private const int WM_NCHITTEST = 0x84;
private const int HTCLIENT = 0x1;
private const int HTCAPTION = 0x2;

protected override void WndProc(ref Message m)
{
    base.WndProc(ref m);

    if (m.Msg == WM_NCHITTEST)
    {
        // Convert HTCLIENT to HTCAPTION
        if (m.Result.ToInt32() == HTCLIENT)
        {
            m.Result = (IntPtr)HTCAPTION;
        }
    }
}

Question 2:

You can create a form of an arbitrary, non-rectangular shape by setting the Region property of your form to the custom Region of your choice. If you have experience with graphics programs like Photoshop, you can think of this as setting a "clipping region" for your form: the window manager will not draw anything outside of the bounds you specify. The pixels in the shape described this Region can even be non-contiguous.

The simplest way to create your region is probably to use the GraphicsPath class, and then use the constructor for the Region class that accepts a single GraphicsPath object as a parameter.

And as I assume you already know, given the first question, you'll have to set the FormBorderStyle property to None to make sure that the default borders drawn by the window manager disappear.

Unfortunately, these regions cannot be anti-aliased. See Hans's answer to this question for more details on these limitations.

Finally, it is worth noting that this latter approach to creating non-rectangular forms can produce some downright ugly user interfaces that don't at all improve the usability of your product, like so:

Windows Media Player "alien head"

Please use this technique sparingly and exercise good judgment. When in doubt, rectangles are actually a really good shape for windows.

thank you cody your answer is very useful to me. I have applied this but my contexMenustrip for the background become disabled. Is there any way that i can have both events for background? — Prince123, Feb 17 '12 at 04:03
You should be aware that using WM_NCHITTEST and HTCAPTION will have side effects. For example: double-clicking the form will cause it to maximize. — AVIDeveloper, Feb 17 '12 at 08:43
I can solve this by disabling MaximizeBox. But how to solve for contexmenustrip that got affect of it. — Prince123, Feb 17 '12 at 16:00
@prince: I would recommend handling the [`WM_CONTEXTMENU` message](http://msdn.microsoft.com/en-us/library/windows/desktop/ms647592.aspx) and showing your menu in response to that message. A common pitfall is doing this with mouse right-click events, but that ignores the possibility of using the specialized keyboard button or other keyboard shortcuts to show context menus. Your right click event won't work for that, `WM_CONTEXTMENU` will. — Cody Gray - on strike, Feb 17 '12 at 17:12
AVIDeveloper is right, this method does have "side effects" (although I question if that's the appropriate term). As I explained, it effectively tells the window manager to treat the client area of your form as if it were part of the caption area. Thus, you'll inherit *all* of the behavior of the caption area, including double-click to maximize, dragging to move, etc. Generally that's what you want, otherwise you'd just listen for `MouseMove` events, check to see if the mouse button is held down, and change the form's `Location` property. — Cody Gray - on strike, Feb 17 '12 at 17:13

AVIDeveloper · Answer 2 · 2012-02-17T09:10:21.680

Here's a link to an impressive CodeProject article, which will answer both your questions: An Alpha Channel Composited Windows Form with Designer Support. The implementation uses a front and back windows combination. It uses WS_EX_LAYERED to display a bitmap-based window, and hooks mouse events to the main form in order to allow dragging it around.

One thing that looks a bit off is when the window is being dragged around - there's a small delay between the movement of the front and back windows, making them look like they're chasing each other.

This next CodeProject article shows a way to overcome that problem by making use of DeferWindowPos: Alpha Blended Windows Forms.

Good luck!

Creating non-rectangular forms that can be moved by dragging anywhere in the background area

2 Answers2

Linked