A partial class is a feature in some object-oriented programming languages that allows the implementation of a class to be split between multiple source code files. Partial classes, which are also called partial types, do not exist in many programming languages, particularly older ones like C++ and Java. Each language that implements partial classes, such as C# and Ruby, handles the implementation a little differently.
In general, partial classes use the "partial" keyword to denote class pieces, and whether the keyword is capitalized depends on the programming language being used. Each piece of the class is initialized in the same way with identical class declarations, and without these "partial" keyword, it would be illegal to create multiple classes in multiple files all with the same name. The "partial" keyword tells the compiler or interpreter, depending on the language, to merge all the classes into a single large unit. All the distinct parts of the class must have the same visibility, be it public, private, or protected. If one part of the class is designated as abstract, the merged class is also designated to be abstract, even if none of the other individual parts were designated as such.
The implementation of a partial class across multiple files has some significant advantages both in programming style and organization. Partial classes allow for explicit separation of related pieces of code into more specialized groups. In addition, a partial class allows multiple programmers to work on specific parts of code without potentially disrupting the others and accidentally overwriting revisions. Even for single programmers, partial classes can be useful in that they can be used to create very large classes without also creating a single unwieldy file that is difficult to organize and read through.
Partial class implementation patterns also has some significant disadvantages. Despite being under the umbrella of object-oriented programming, partial classes actually violate the encapsulation principles central to object-oriented programming. In object-oriented programming, a class is supposed to be a single unified entity which has its own unique states and behaviors, is self-contained, and is able to be used as a cohesive unit. Partial classes violate this idea because each partial class acts like a single separate class. Although the pieces that make up a partial class are merged into one large class at runtime, the partial class's pieces are often independent entities that could be used on their own without any form of merging necessary.