Is it a bad practice to have multiple classes in the same file?

Question

I used to have one class for one file. For example car.cs has the class car. But as I program more classes, I would like to add them to the same file. For

Answer 1

In Java, one public class per file is the way the language works. A group of Java files can be collected into a package.

In Python, however, files are "modules", and typically have a number of closely related classes. A Python package is a directory, just like a Java package.

This gives Python an extra level of grouping between class and package.

There is no one right answer that is language-agnostic. It varies with the language.

Answer 2

One class per file is a good rule, but it's appropriate to make some exceptions. For instance, if I'm working in a project where most classes have associated collection types, often I'll keep the class and its collection in the same file, e.g.:

public class Customer { /* whatever */ }

public class CustomerCollection : List<Customer> { /* whatever */ }

The best rule of thumb is to keep one class per file except when that starts to make things harder rather than easier. Since Visual Studio's Find in Files is so effective, you probably won't have to spend much time looking through the file structure anyway.

Answer 3

If you are working on a team, keeping classes in separate files make it easier to control the source and reduces chances of conflicts (multiple developers changing the same file at the same time). I think it makes it easier to find the code you are looking for as well.

Answer 4

The Smalltalk answer is: you should not have files (for programming). They make versioning and navigation painful.

Answer 5

It can be bad from the perspective of future development and maintainability. It is much easier to remember where the Car class is if you have a Car.cs class. Where would you look for the Widget class if Widget.cs does not exist? Is it a car widget? Is it an engine widget? Oh maybe it's a bagel widget.

Answer 6

The only time I consider file locations is when I have to create new classes. Otherwise I never navigate by file structure. I Use "go to class" or "go to definition".

I know this is somewhat of a training issue; freeing yourself from the physical file structure of projects requires practice. It's very rewarding though ;)

If it feels good to put them in the same file, be my guest. Cant do that with public classes in java though ;)