C#/.NET user32.dll

Получение координат экрана:

[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));
    }
}

Конвертирование пикселей в WPF единицы:

[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;          
}

Собираем всё вместе:

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));
    }
}