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c#.netextension-methods
Matt
Matt

Reputation: 27001

Should extension methods only be used on classes whose code you don't have access to?

Should extension methods only be used on classes whose code you don't have access to?

I'm struggling to come up with a reason to have extension methods versus making it partial and adding the classes in an external file.

My specific scenario is as follows: I have classes that represent entities in a database via EF. I'm debating making the classes it renders partial and adding my own methods. Are extension methods a more valid alternative approach or are they not intended to be used when you have access to the code of the class you are extending?

Upvotes: 2

Views: 597

Answers (5)

Bjorn Coltof
Bjorn Coltof

Reputation: 509

Apart from the ability to provide extensions to interfaces, I use extension methods to add 'members' to a class that work using only the class' public interface. So if a method needs access to a private/protected member, it will become a class member, if not an extension method. This keeps the classes themselves small and focused...

Upvotes: 2

Konstantin Oznobihin
Konstantin Oznobihin

Reputation: 5327

You might want to look at How Non-Member Functions Improve Encapsulation. There are some C++ specifics, but main idea applies to other OO languages as well. In short: the less methods you have in a class the easier it is to understand who and how changes private class state.

Upvotes: 0

Steve Wellens
Steve Wellens

Reputation: 20640

No.

You may have a class that works fine in ninety percent of your projects. By adding an extension method you don't 'pollute' the original class but can still leverage it in the other ten percent of your projects.

Upvotes: 1

Eben Roux
Eben Roux

Reputation: 13256

Should extension methods only be used on classes whose code you don't have a access to?

Not necessarily. There are situations where extension methods can still help. I recently had an issue with the xml serializer where it could serialize an object that had a method that made use of a linq / lamba expression. Moving the method to an extension method resolved that. I like to use extension methods on DTOs also.

Upvotes: 0

Anthony Pegram
Anthony Pegram

Reputation: 126952

The canonical counter-example is extension methods on an interface, as even if you control the source, there is no implementation. See: Linq.

But, yes, generally speaking, if you control the source of the concrete class, it is not unreasonable to expect to add the functionality to the class directly rather than using an extension method, if it makes sense for the functionality to actually be part of the class instance.

On the other hand, and coming back to your situation, you might consider neither approach. Your entities are data models, I would not add methods to those models, but rather encapsulate that functionality elsewhere. Those models exist to encapsulate your data, logic that might operate with or against that data might be better served in a different unit. But that really depends upon what your methods are doing, and also assuming they're not something like a trivial wrapper over one or more properties, for example.

Upvotes: 4

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