c# – When to create your own dll what should be in it – Education Career Blog

I as of yet, have not had a need to create a dll in .NET as my programs tend to be distinct from one another (and I haven’t programmed for too long). However I am starting to gather a library of regularly used methods now which tend to have copies in lots of my programs (or similar methods that could be made more generic).

As I understand it a dll should be created to share common methods between programs.

So I can put my methods in the dll, what about things like custom dialog boxes and common forms. I currently have a custom error form and a print preview dialog which are used in almost all my programs. Is it good practice to put these in a dll?


“Good practice” in this case really relies on how much code-duplication you’re cutting down on. If these dialogues are used throughout a lot of your programs and they take up a lot of code, then yes, it’s probably a good idea to move them to a shared space.

But don’t be tempted to throw everything in there. You’ll find yourself having to override it and by that point you’re probably writing more code than you were before. Keep things simple. Keep them generic. Keep them useful.

Before you start, though, be aware that you are creating a dependency tree. If you update your library chances are you’ll have to put in some time maintaining your apps that use it… But this is no different from using a third-party lib.


To create a new dll, you just add new project (library) to your solution, and add it as a project reference to your main programme.

It is possible to put anything you want to this project. All dialog boxes, user controls, etc. are just classes, so you can share them as easily as any other class. I think it is good practise to share as much as possible.


Sure why not?

What you’re building here is actually a small framework, pretty much like the .Net framework itself. Everything that you think is common between your applications can be put in the assemblies: forms, methods, business logic, exceptions, common data access.

When your framework grows you might want to split up that common DLL. For example if you have common forms in a DLL and you also develop batch applications then they don’t need to reference to the DLL containing the WinForms specific classes.


Alternatively you can put the source files for these methods in a common place and just add them as links when you include them in your project/solution.

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