Partial class across projects is NOT ALLOWED

Well, you learn something new everyday and today I found out that you can’t have a partial class that spans across projects in C#, i.e. if I have a partial class called MyClass in ProjectA:

   1: namespace ProjectA.Entities

   2: {

   3:     public partial class MyClass { }

   4: }

   5:     

I won’t be able to extend it by defining another partial class called MyClass in ProjectB even if I put the new partial class under the same namespace. Now, that’s a bummer.. but when you think about it it actually makes perfect sense!

If partial class across projects were to be allowed, it would represent a huge security risk because it’d allow potentially harmful code to be injected into existing binaries. Imagine if I had a partial class with private methods that I don’t want to expose:

   1: public partial class MyClass

   2: {

   3:     // public methods, etc. etc.

   4:     // ...

   5:  

   6:     // no one else should be allowed to call this method!!!

   7:     private void DoSecretHandshake()

   8:     {

   9:         ...

  10:     }

  11: }

I kept these methods private so others can call them, so then I compile, build and distribute the binaries, and all of a sudden some evil-doer is able to extend my class and do this:

   1: public partial class MyClass

   2: {

   3:     // expose the not so secret handshake

   4:     public void HackSecretHandshake()

   5:     {

   6:         DoSecretHandshake();    // mwahahahaha

   7:     }

   8: }

Now that’s no good! OK.. if someone really wants to execute the DoSecretHandshake method, they can always use reflection to do it, but at least it won’t be so easy and indeed tempting for them to try it..

Oh, and one more thing, a partial class is a compile time only piece of syntactic sugar and the whole thing is still compiled into a single type, in a single project. It does however, allow you to separate a huge class file into multiple files and hence making it easier to organise and manage.

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter


Hi, I’m Yan. I’m an AWS Serverless Hero and the author of Production-Ready Serverless.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.


Check out my new course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. Including basic concepts, HTTP and event triggers, activities, callbacks, nested workflows, design patterns and best practices.

Get Your Copy