Because of the distributed nature of web applications, it is not practical to rely on a reference to a single document object for the lifetime of a viewer. Instead, the PageView Class and ThumbnailView Class controls introduce the concept of a document identifier.

Using document identifiers instead of .NET objects to represent a document allows ImageGear to be used in a web form environment and allows an ImageGear client to continue functioning if the server that created it is rebooted.

A document identifier is a string that uniquely identifies the document, via relative path to a file, or via a database key. 

To specify a document to view, set the PageView.DocumentIdentifier Property and ThumbnailView.DocumentIdentifier Property. (The PageView.PageNumber Property and ThumbnailView.PageNumber Property indicate the page to view in a multipage document.) To get the encoded value of the document identifier, use the PageView.EncodedDocumentIdentifier Property and ThumbnailView.EncodedDocumentIdentifier Property.

The ImGearThumbnailList class control does not contain DocumentIdentifier property but the documentidentifier is used for data binding. The client side ImageGear.Web.UI.ImGearDocument contains a DocumentIdentifier property. Therefore, internally, the server control uses the bound DocumentIdentifier to create a Client side ImGearDocument object.