Introducing C# 9: Extending Partial Methods

Introduction

C# 8 (and above) has some restrictions regarding partial methods. For example :

• Partial methods must have a void return type
• Partial methods can’t have out parameters
• Partial methods can’t have any accessibility keyword (public, private, protected etc….)

C# 9 aims to remove these restrictions. If you want to learn more about the motivation behind this, you can find a good description on the Github page here: https://github.com/jaredpar/csharplang/blob/partial/proposals/extending-partial-methods.md

Behavior before C# 9

Below are some examples of what happens when a partial methods has or not an accessibility keyword, has or not an out parameter, a void or not return type, implements an interface:

Note for interface implementation: because C# 8 and above doesn’t support accessibility keyword on partial methods it’s impossible to implement partial method from its interface signature, because without any keyword, the method is implicitly a private method, which doesn’t allow to implement this method from its interface signature.

Behavior with C# 9

Now let’s take the exact same partial class definition above and see what’s the compiler behavior now:

C# 9 allows now what was missing in C# 8 and above, but, it requires now an implementation on methods that are defined with:

• void or not return type
• out parameters
• accessibility keyword (public, private, protected etc….)

As you can see above C# 9 brings a new error code with its message if the previous conditions are not fulfilled:

CS8795: Partial method must have an implementation part because it has accessibility modifiers

I’m really excited by what’s coming with C# 9, and you ? 😉