Moves a specified Window to the center of a monitor where the cursor locates.

WindowHelper.cs

using System;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Interop;

public static class WindowHelper
{
    /// <summary>
    /// Moves a specified Window to the center of a monitor where the cursor locates.
    /// </summary>
    /// <param name="window">Window to be moved</param>
    /// <returns>
    /// This should be called before Loaded event such as ArrangeOverride method.
    /// </returns>
    public static bool MoveToCenter(Window window)
    {
        if (window.WindowStartupLocation != WindowStartupLocation.CenterScreen)
            return false;

        if (window.IsLoaded)
            return false;

        return MoveToCenterBase(window);
    }

    public static bool MoveToCenterBase(Window window)
    {
        if (!GetCursorPos(out POINT cursorPoint))
            return false;

        var monitorHandle = MonitorFromPoint(
            cursorPoint,
            MONITOR_DEFAULTTO.MONITOR_DEFAULTTONULL);
        if (monitorHandle == IntPtr.Zero)
            return false;

        var monitorInfo = new MONITORINFO() { cbSize = (uint)Marshal.SizeOf<MONITORINFO>() };
        if (!GetMonitorInfo(
            monitorHandle,
            ref monitorInfo))
        {
            return false;
        }

        var windowHandle = new WindowInteropHelper(window).EnsureHandle();
        if (windowHandle == IntPtr.Zero)
            return false;

        if (!GetWindowPlacement(
            windowHandle,
            out WINDOWPLACEMENT windowPlacement))
        {
            return false;
        }

        int left = monitorInfo.rcWork.left + Math.Max(0, (int)((monitorInfo.rcWork.Width - windowPlacement.rcNormalPosition.Width) / 2D));
        int top = monitorInfo.rcWork.top + Math.Max(0, (int)((monitorInfo.rcWork.Height - windowPlacement.rcNormalPosition.Height) / 2D));

        windowPlacement.rcNormalPosition = new RECT(left, top, width: windowPlacement.rcNormalPosition.Width, height: windowPlacement.rcNormalPosition.Height);

        return SetWindowPlacement(
            windowHandle,
            ref windowPlacement);
    }

    #region Win32

    [DllImport("User32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool GetCursorPos(
        out POINT lpPoint);

    [DllImport("User32.dll")]
    private static extern IntPtr MonitorFromPoint(
        POINT pt,
        MONITOR_DEFAULTTO dwFlags);

    private enum MONITOR_DEFAULTTO : uint
    {
        MONITOR_DEFAULTTONULL = 0x00000000,
        MONITOR_DEFAULTTOPRIMARY = 0x00000001,
        MONITOR_DEFAULTTONEAREST = 0x00000002,
    }

    [DllImport("User32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool GetMonitorInfo(
        IntPtr hMonitor,
        ref MONITORINFO lpmi);

    [StructLayout(LayoutKind.Sequential)]
    private struct MONITORINFO
    {
        public uint cbSize;
        public RECT rcMonitor;
        public RECT rcWork;
        public uint dwFlags;
    }

    [DllImport("User32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool GetWindowPlacement(
        IntPtr hWnd,
        out WINDOWPLACEMENT lpwndpl);

    [DllImport("User32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool SetWindowPlacement(
        IntPtr hWnd,
        [In] ref WINDOWPLACEMENT lpwndpl);

    [StructLayout(LayoutKind.Sequential)]
    private struct WINDOWPLACEMENT
    {
        public uint length;
        public uint flags;
        public uint showCmd;
        public POINT ptMinPosition;
        public POINT ptMaxPosition;
        public RECT rcNormalPosition;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct POINT
    {
        public int x;
        public int y;
    }

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

        public int Width => right - left;
        public int Height => bottom - top;

        public RECT(int x, int y, int width, int height)
        {
            left = x;
            top = y;
            right = x + width;
            bottom = y + height;
        }
    }

    #endregion
}