How to use Win32 GetMonitorInfo() in .NET c#?

Question

I have to implement a feature where the last position of the window is saved. When the application starts up this position needs to be obtained and restored.

Now it could be that a second monitor is dismantled. If the last position is on a now non-visible monitor (in other words the saved coordinates are outside the visible coordinates), this case should be caught and the coordinates shall be set to the default rather than last position.

In order to retrieve the information about monitors I need to use Win32. It is not easy for me to make this work.

I have created a Helper CLass:

public static class DisplayHelper
    {
        private const int MONITOR_DEFAULTTONEAREST = 2;

        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        public static extern int GetSystemMetrics(int nIndex);

        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        private static extern UInt32 MonitorFromPoint(Point pt, UInt32 dwFlags);

        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        private static extern bool GetMonitorInfo(UInt32 monitorHandle, ref MonitorInfo mInfo);


        public static void GetMonitorInfoNow(MonitorInfo mi, Point pt)
        {
            UInt32 mh = MonitorFromPoint(pt, 0);
            mi.cbSize = (UInt32)System.Runtime.InteropServices.Marshal.SizeOf(typeof(MonitorInfo));
            mi.dwFlags = 0;
            bool result = GetMonitorInfo(mh, ref mi);

        }
    }

And these are my attempts to create the MonitorInfo and Rect classes:

[StructLayout(LayoutKind.Sequential)]
    public class MonitorInfo
    {
        public UInt32 cbSize;
        public Rectangle2 rcMonitor;
        public Rectangle2 rcWork;
        public UInt32 dwFlags;

        public MonitorInfo()
        {
            rcMonitor = new Rectangle2();
            rcWork = new Rectangle2();

            cbSize = (UInt32)System.Runtime.InteropServices.Marshal.SizeOf(typeof(MonitorInfo));
            dwFlags = 0;
        }
    }

    [StructLayout(LayoutKind.Sequential)]
    public class Rectangle2
    {
        public UInt64 left;
        public UInt64 top;
        public UInt64 right;
        public UInt64 bottom;

        public Rectangle2()
        {
            left = 0;
            top = 0;
            right = 0;
            bottom = 0;
        }
    }

I am using this code like this to obtain the visible monitors:

//80 means it counts only visible display monitors.
int lcdNr = DisplayHelper.GetSystemMetrics(80);
var point = new System.Drawing.Point((int) workSpaceWindow.Left, (int) workSpaceWindow.Top);
MonitorInfo monitorInfo = new MonitorInfo();
DisplayHelper.GetMonitorInfoNow(monitorInfo, point);

The last method throws an exception when trying to execute

bool result = GetMonitorInfo(mh, ref mi);

Any suggestions what I need to do to fix this?

Ossein

Answer 1

Rather than calling a native API, you should use System.Windows.Forms.Screen. It should have everything you need, and be much easier to use.

Screen.FromPoint is the managed equivalent of your GetMonitorInfoNow function with the MONITOR_DEFAULTTONEAREST option. I just noticed you aren't using that option, so you may have to write your own or use the correct P/Invoke signatures.

Writing your own should be fairly simple, if you just reference System.Drawing and System.Windows.Forms. Both of these should work:

static Screen ScreenFromPoint1(Point p)
{
    System.Drawing.Point pt = new System.Drawing.Point((int)p.X, (int)p.Y);
    return Screen.AllScreens
                    .Where(scr => scr.Bounds.Contains(pt))
                    .FirstOrDefault();
}

static Screen ScreenFromPoint2(Point p)
{
    System.Drawing.Point pt = new System.Drawing.Point((int)p.X, (int)p.Y);
    var scr = Screen.FromPoint(pt);
    return scr.Bounds.Contains(pt) ? scr : null;
}

If you prefer to make the Win32 calls yourself, the proper P/Invoke signatures (i.e. what you'd get from decompiling the .Net DLL) for the functions you need to call are:

    [DllImport("User32.dll", CharSet=CharSet.Auto)] 
    public static extern bool GetMonitorInfo(HandleRef hmonitor, [In, Out]MONITORINFOEX info);
    [DllImport("User32.dll", ExactSpelling=true)]
    public static extern IntPtr MonitorFromPoint(POINTSTRUCT pt, int flags);

    [StructLayout(LayoutKind.Sequential,CharSet=CharSet.Auto, Pack=4)]
    public class MONITORINFOEX { 
        public int     cbSize = Marshal.SizeOf(typeof(MONITORINFOEX));
        public RECT    rcMonitor = new RECT(); 
        public RECT    rcWork = new RECT(); 
        public int     dwFlags = 0;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst=32)] 
        public char[]  szDevice = new char[32];
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct POINTSTRUCT { 
        public int x;
        public int y;
        public POINTSTRUCT(int x, int y) {
          this.x = x; 
          this.y = y;
        } 
    } 

    [StructLayout(LayoutKind.Sequential)] 
    public struct RECT {
        public int left; 
        public int top; 
        public int right;
        public int bottom; 
    }

Mohun

Thanks Gabe, I have forgotten to mention my Application is written in WPF. Theoretically I would have no access to System.Windows.Forms.Screen, unless it would also work with WPF? – Ossein

@Kave: Just add a reference to System.Windows.Forms.dll and you can use it from WPF. – Mohun

@Mohun - It might be easier, but I'm not sure I'd load a 5MB assembly for one class/method even with any optimizations. Not to mention it's dependencies ;-) – Cursed

Thanks Gabe, yes I got it now there. But I don't grasp how this should work. Screen test = Screen.FromPoint(new Point(2561, 400)); Shouldn't really give me a screen object as my max X is 2560. Yet I get a screen object with X 1280, Y 0 , Width 1280 and height 1024. I dont see any indication how my point coordinates would be outside the screen. Any idea? Thanks – Ossein

@Kave: I see, you aren't passing in the MONITOR_DEFAULTTONEAREST option. See my edit for how to get around that. – Mohun

"It should have everything you need" It does not have the Screen positions relative to themselves – Holster

Answer 2

I found one different is
public static extern bool GetMonitorInfo(IntPtr hMonitor, [In,Out] MONITORINFO lpmi) and
public static extern bool GetMonitorInfo(IntPtr hMonitor, ref MONITORINFO lpmi)

In my case, the ref keywork made the function always return false.
But if remove this keyword or usr [In,Out], it work.

More info about ref vs. [In,Out] on This.

Neolamarckism

You need [Out] if you have a class, not a struct. – Tops

Answer 3

Your Rectangle2 should use Int32 or just int, not Int64. More information can be found here.

Also it needs to be a struct, not a class. Same goes for your MonitorInfo class (it should be a struct). I'd recommend trying the version from the link above, or compare them with your versions.

Cursed

Answer 4

Since you are using the MonitorInfo class, not a struct, you must specify the [Out] attribute and not use ref in order for the marshaler to update your class correctly.

[DllImport("user32", CharSet = CharSet.Auto, SetLastError = true)]
internal static extern bool GetMonitorInfo(
    IntPtr hmonitor, 
    [In, Out] MonitorInfo info);

You can also go to using structure and ref, then it will look like this:

[DllImport("user32", CharSet = CharSet.Auto, SetLastError = true)]
internal static extern bool GetMonitorInfo(
    IntPtr hmonitor, 
    ref MonitorInfoEx info);

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Pack = 4)]
internal struct MonitorInfoEx
{
   public uint cbSize;
   public RECT rcMonitor;
   public RECT rcWork;
   public uint dwFlags;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
   public char[] szDevice;
}

var info = new MonitorInfoEx
{
    cbSize = (uint)Marshal.SizeOf(typeof(MonitorInfoEx)),
};
GetMonitorInfo(hDesktop, ref info);

Tops