Creating a zoom factor extension that works with a ToolbarControl
Extension\IZoomExtension.cs
// Copyright 2010 ESRI
// 
// All rights reserved under the copyright laws of the United States
// and applicable international laws, treaties, and conventions.
// 
// You may freely redistribute and use this sample code, with or
// without modification, provided you include the original copyright
// notice and use restrictions.
// 
// See the use restrictions.
// 

//IZoomExtension interface
public interface IZoomExtension
{
  double ZoomFactor
  {
    get;
    set;
  }
}

public class myIZoomExtension: IZoomExtension
{
  public double ZoomFactor 
  {
    get 
    {
      return 0;
    }

    set 
    {
    }
  }
}