partial (Type) (C# Reference) Partial type definitions allow for
the definition of a class, struct, or interface to be split into
multiple files. In File2.cs the declaration: Splitting a class,
struct or interface type over several files can be useful when
you are working with large projects, or with automatically
generated code such as that provided by the Windows Forms
Designer. A partial type may contain a partial method. For more
information, see Partial Classes and Methods (C# Programming
Guide). For more information, see the C# Language Specification.
The language specification is the definitive source for C#
syntax and usage.