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

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; 
    }

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.