How to get mouse position related to desktop in WPF?

Question

Problem

When you search for such question using google you get a lot of hits but all solutions assume you have at least one window.

But my question is just like I phrased it -- not assumptions at all. I can have a window, but I could have zero windows (because I didn't even show one or I just closed the last one). So in short the solution cannot rely on any widget or window -- the only thing is known, is there is a desktop (and app running, but it does not have any windows).

So the question is -- how to get the mouse position?

Background

I would like to show windows centered to mouse position. There is no such mode in WPF (there are only center to owner, or center to screen) so I have to do it manually. The missing piece is mouse position.

Edits

Thank you all, so now I have the first part of the solution -- raw position. Now there is a problem how to convert the data for WPF. I found such topic: http://stackoverflow.com/questions/1189384/wpf-pixels-to-desktop-pixels but again, it assumes having some window.

Then I googled more and I found solution: http://jerryclin.wordpress.com/2007/11/13/creating-non-rectangular-windows-with-interop/

the code includes class for scaling up/down coordinates relying only on info about desktop. So joining those two pieces, I finally get the solution :-). Thanks again.

Answer 1

Two options:

Use System.Windows.Forms.Control.MousePosition, or p/invoke

[DllImport("user32.dll", CharSet=CharSet.Auto, ExactSpelling=true)]
public static extern bool GetCursorPos([In, Out] NativeMethods.POINT pt);

The first option already does the p/invoke for you. I'm not entirely sure it requires you have some UI splashed up, but I don't think so. Yes, its winforms and not wpf, but it really doesn't have anything to do with where its located at.

If you want to skip any dependencies on system.windows.forms.dll then check out more information about the second on pinvoke.net.

Answer 2

Getting the Screen Coordinates:

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool GetCursorPos(out POINT lpPoint);

[StructLayout(LayoutKind.Sequential)]
public struct POINT
{
    public int X;
    public int Y;

    public POINT(int x, int y)
    {
        this.X = x;
        this.Y = y;
    }
}

private void WritePoint(object sender, RoutedEventArgs e)
{
    POINT p;
    if (GetCursorPos(out p))
    {
        System.Console.WriteLine(Convert.ToString(p.X) + ";" + Convert.ToString(p.Y));
    }
}

Converting Pixels to WPF Units:

[DllImport("User32.dll")]
static extern IntPtr GetDC(IntPtr hwnd);

[DllImport("gdi32.dll")]
static extern int GetDeviceCaps(IntPtr hdc, int nIndex);

[DllImport("user32.dll")]
static extern bool ReleaseDC(IntPtr hWnd, IntPtr hDC);

private Point ConvertPixelsToUnits(int x, int y)
{
    // get the system DPI
    IntPtr dDC = GetDC(IntPtr.Zero); // Get desktop DC
    int dpi = GetDeviceCaps(dDC, 88);
    bool rv = ReleaseDC(IntPtr.Zero, dDC);

    // WPF's physical unit size is calculated by taking the 
    // "Device-Independant Unit Size" (always 1/96)
    // and scaling it by the system DPI
    double physicalUnitSize = (1d / 96d) * (double)dpi;
    Point wpfUnits = new Point(physicalUnitSize * (double)x,
        physicalUnitSize * (double)y);

    return wpfUnits;          
}

Putting both together:

private void WriteMouseCoordinatesInWPFUnits()
{
    POINT p;
    if (GetCursorPos(out p))
    {
        Point wpfPoint = ConvertPixelsToUnits(p.X, p.Y);
        System.Console.WriteLine(Convert.ToString(wpfPoint.X) + ";" + Convert.ToString(wpfPoint.Y));
    }
}