MDI (Multiple Document Interface) is a Microsoft Windows programming interface for creating an application that enables users to work with multiple documents at the same time. Each document is in a separate space with its own controls for scrolling. The user can see and work with different documents such as a spreadsheet, a text document, or a drawing space by simply moving the cursor from one space to another.

An MDI application is something like the Windows desktop interface since both include multiple viewing spaces. However, the MDI viewing spaces are confined to the application's window or client area . Within the client area, each document is displayed within a separate child window . MDI applications can be used for a variety of purposes - for example, working on one document while referring to another document, viewing different presentations of the same information, viewing multiple Web sites at the same time, and any task that requires multiple reference points and work areas at the same time

The .NET Framework 3.5 provides the foundation for building connected and appealing applications, productively, on a wide variety of systems from the device to the data center.

Microsoft continues to support developers using the .NET Framework platform by providing support for applications built in previous versions to the 3.5 release, so existing applications built for .NET Framework 2.0 or .NET Framework 3.0 will continue to work.

• Deep integration of Language Integrated Query (LINQ) and data awareness.
• Support for Web 2.0 AJAX-style applications and services in ASP.NET and WCF.
• Full tooling support for WF, Windows Communication Foundation (WCF), and Windows Presentation Foundation (WPF), including the new workflow-enabled services technology.
• New classes in the base class library (BCL) for the .NET Framework 3.5 address the most common customer requests.

• Presentation Layer
• Business Logic Layer
• Data Access Layer

In most of the cases the data entered by the end user needs some kind of validation or further processing. This is the responsibility of Business Logic Layer.

Finally, your application data needs to be stored and retrieved in some data store (RDBMS, XML etc.). This task is handled by Data Access Layer.

In short, the process works like this:

• User requests for some application data.
• The data access layer retrieves the data and is forwarded to the presentation layer via business logic layer. Sometimes data access layer gives this data directly to presentation layer.
• Presentation layer receives the data to be displayed via business logic layer.
• The user changes the data and initiates the appropriate action (such as insert, or update).
• The business logic layer validates the data submitted by the user.
• If the data is valid it is handed over to data access layer for updating into the database.

• The applications gets divided in logically isolated pieces reducing tight coupling between the UI, business processes and database.
• Change in the underlying database and data access methods do not have any effect on the presentation layer or client application.
• Client application no longer has SQL statements embedded in it. This makes it de-coupled from rest of the application.
• Table and column names can be effectively eliminated from the client-side code.
• The client application is unaware from where data comes (location transparency).
• It becomes easier to modify or extend your application, without breaking or recompiling the client-side code.