A partial class in C# is a feature that allows a single class to be split into multiple files. This can be useful in large projects where multiple programmers are working on the same class at the same time, as it allows them to work on different parts of the class in separate files. When the application is compiled, all the parts of the class are combined into a single class file.

Some key points to remember about partial classes in C# include:

• All partial classes must contain the partial keyword.
• All parts of a partial class must have the same name and be in the same assembly and namespace.
• All parts of a partial class must have the same accessibility modifier, such as public or private.
• If any part of a partial class is declared abstract, sealed, or has a base type, then the whole class is declared of the same type.
• Different parts of a partial class can have different base types, and the final class will inherit all the base types.
• The partial modifier can only appear immediately before the keywords class, struct, or interface.

Partial classes can also contain partial methods, which are methods split into two separate files of the partial class. One file contains the signature of the method, while the other file contains an optional implementation of the partial method. Both the declaration and implementation of a method must have the partial keyword.

Overall, partial classes in C# can make it easier to work with large classes and can help with code organization.