Partial Classes are one of the enhancements in .NET 2.0
Partial classes mean that your class definition can be split into multiple physical files. Logically, partial classes do not make any difference to the compiler. During compile time, it simply groups all the various partial classes and treats them as a single entity.
One of the greatest benefits of partial classes is that it allows a clean separation of business logic and the user interface (in particular the code that is generated by the visual designer). Using partial classes, the UI code can be hidden from the developer, who usually has no need to access it anyway. Partial classes will also make debugging easier, as the code is partitioned into separate files.
While partial classes allow you to split the definition of a class into multiple files, you cannot mix languages. That is, all partial classes must be written in the same language. Besides using the "partial" keyword for classes, you can also use it for structures and interfaces.
Here are some good reasons to use partial classes:
1.They allow programmers on your team to work on different parts of a class without needing to share the same physical file. While this is useful for projects that involve big class files, be wary: If you find your class files getting too large, it may well signal a design fault and refactoring may be required.
2.The most compelling reason for using partial class is to separate your application business logic from the designer-generated code. For example, the code generated by Visual Studio 2005 for a Windows Form is kept separate from your business logic (we will discuss this in a later section). This will prevent developers from messing with the code that is used for the UI. At the same time, it will prevent you from losing your changes to the designer-generated code when you change the UI.