A structure is a type, much like a class. As such, it should be defined in the same way that a class is defined, i.e. in its own code file. I generally right-click the project in the Solution Explorer and select the New Class item, then simply change the 'class' keywords to 'structure'. It is possible to declare multiple types in the same code file or even inside other types but, while there can be legitimate reasons to do so, you should generally be declaring each class and structure in its own code file.