IWin32Window implementation to allow VSTO addins to access Excel's main window.

ThisAddin.cs

using System.Reflection;
using System.Windows.Forms;

namespace MyExcelAddin
{
    public partial class ThisAddIn
    {

		public Win32Window MainWindow { get; set; }

		public string AssemblyTitle
		{
			get {
				try
				{
					var attr = typeof(ThisAddIn)
						.GetTypeInfo()
						.Assembly
						.GetCustomAttribute(typeof(AssemblyTitleAttribute))
						as AssemblyTitleAttribute;
					return attr?.Title ?? "Excel";
				}
				catch
				{
					return "Excel";
				}
			}
		}

		private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
			this.MainWindow = new Win32Window(Globals.ThisAddIn.Application.Hwnd);
        }
		
	}
}

Win32Window.cs

using System;
using System.Runtime.InteropServices;
using System.Drawing;

namespace MyExcelAddin
{
	public class Win32Window: System.Windows.Forms.IWin32Window
	{

		/// <summary>
		/// Retrieves the dimensions of the bounding rectangle of the specified window. 
		/// The dimensions are given in screen coordinates that are relative to the upper-left corner of the screen.
		/// </summary>
		/// <see cref="https://msdn.microsoft.com/en-us/library/windows/desktop/ms633519(v=vs.85).aspx"/>
		[DllImport("user32.dll")]
		[return: MarshalAs(UnmanagedType.Bool)]
		static extern bool GetWindowRect( HandleRef hWnd, out RECT lpRect );

		/// <summary>
		/// The RECT structure defines the coordinates of the upper-left and lower-right corners of a rectangle.
		/// </summary>
		/// <see cref="https://msdn.microsoft.com/en-us/library/windows/desktop/dd162897(v=vs.85).aspx"/>
		[StructLayout(LayoutKind.Sequential)]
		public struct RECT
		{
			/// <summary>The x-coordinate of the upper-left corner of the rectangle.</summary>
			public int Left;
			/// <summary>The y-coordinate of the upper-left corner of the rectangle.</summary>
			public int Top;
			/// <summary>The x-coordinate of the lower-right corner of the rectangle.</summary>
			public int Right;
			/// <summary>The y-coordinate of the lower-right corner of the rectangle.</summary>
			public int Bottom;
		}

		/// <summary>
		/// Window Handle
		/// </summary>
		IntPtr _hwnd; 

		/// <summary>
		/// Initializes a new instance of the Win32Window object.
		/// </summary>
		/// <param name="hwnd"></param>
		public Win32Window( int hwnd )
		{
			this._hwnd = new IntPtr(hwnd);
		}

		/// <summary>
		/// Gets the Y-coordinate of the bottom edge of the window.
		/// </summary>
		public int Bottom
		{
			get { return this.Rectangle.Bottom; }
		}

		/// <summary>
		/// Window Handle
		/// </summary>
		public IntPtr Handle
		{
			get { return this._hwnd; }
		}

		/// <summary>
		/// Window Handle Reference
		/// </summary>
		private HandleRef HandleRef
		{
			get { return new HandleRef(this,this.Handle); }
		}

		/// <summary>
		/// Gets the height of the window.
		/// </summary>
		public int Height
		{
			get { return this.Rectangle.Width; }
		}

		/// <summary>
		/// Gets the X-coordinate of the left edge of the window.
		/// </summary>
		public int Left
		{
			get { return this.Rectangle.Left; }
		}

		/// <summary>
		/// Gets the Windows Rectangle
		/// </summary>
		public Rectangle Rectangle
		{
			get {
				RECT rect;
				GetWindowRect(this.HandleRef,out rect);
				return new Rectangle(rect.Left,rect.Top,rect.Right-rect.Left,rect.Bottom-rect.Top);
			}
		}

		/// <summary>
		/// Gets the X-coordinate of the right edge of the window.
		/// </summary>
		public int Right
		{
			get { return this.Rectangle.Right; }
		}

		/// <summary>
		/// Gets the Y-coordinate of the top edge of the window.
		/// </summary>
		public int Top
		{
			get { return this.Rectangle.Top; }
		}

		/// <summary>
		/// Gets the width of the window.
		/// </summary>
		public int Width
		{
			get { return this.Rectangle.Width; }
		}

	}
}