Extent

IActiveView.Extent Property

The Extent property holds the visible area of the view.  The Extent is automatically captured whenever the view's transformation changes.  For example, when the active view is a Map, the Extent is modified whenever the spatial reference changes.  Also, the Extent is set the first time a layer is added to a Map.  In layout view, the Extent is initially set to the page size.  In both layout view and data view, zooming in our out changes the Extent. 

The Extent is always adjusted to fit the device frame and it is persisted in the map document.

也就说，我们得到的是可见范围的Envelpe，注意的是,这个Envelope可是实际地图单位的。

 

IMapControl2.Extent Property

Returns or sets a new envelope which is represents the visible extent of the Map. Setting the Extent property will automatically cause the MapControl to refresh its display.

感觉跟上面的东西一样

IActiveView.FullExtent Property

The full extent of the view. In ArcMap's data view, the full extent is the full extent of all layers and it is automatically calculated whenever the spatial reference changes and when layers are added or removed with AddLayer, AddLayers, or DeleteLayer. In layout view, the full extent is the page size expanded some and it is automatically set whenever the page size changes.

IMapControl2.FullExtent Property

Returns or sets a new envelope object representing the full extent of all the data layers in the Map. The FullExtent property is a shortcut to IDisplayTransformation::Bounds. Setting the FullExtent triggers the IMapControlEvents2::OnFullExtentUpdated event.

 

而FullExtent则是整张地图的Envelope