C#

using System;

namespace Net.SourceForge.Koogra.Excel
{
	/// <summary>
	/// Interface that must be implemented by all Excel objects.
	/// </summary>
	/// <remarks>
	/// Basically, Excel objects are simply objects that are inside a workbook.
	/// </remarks>
	public interface IExcelObject
	{
		/// <summary>
		/// Must return a reference to the objects workbook and must not return null.
		/// </summary>
		Workbook Workbook { get; }
	}
}

namespace Net.SourceForge.Koogra.Excel
{
	/// <summary>
	/// Base class implemenation of IExcelObject
	/// </summary>
	public class ExcelObject : IExcelObject
	{
		private Workbook _wb;

		/// <summary>
		/// Constructor.
		/// </summary>
		/// <param name="wb">The workbook.</param>
		/// <exception cref="ArgumentNullException">Exception is thrown if wb is null.</exception>
		public ExcelObject(Workbook wb)
		{
			if(wb == null)
				throw new ArgumentNullException("wb");

			_wb = wb;
		}

		/// <summary>
		/// The workbook.
		/// </summary>
		public Workbook Workbook
		{
			get
			{
				return _wb;
			}
		}
	}
}

[classname] : [interface]

com visibility is not set on any type

using System.Runtime.InteropServices

[assembly: ComVisible(true)]

// inside Form1
string myVar;
...
public string MyVar
{
   get
   {
      return myVar;
   }
}

class DoWork
{
    private Form1 form;
    public DoWork(Form1 formInstance)
    {
       this.form = formInstance;
       string theVar = this.form.MyVar;
    }
}