Introducing C# 9: Extending Partial Methods
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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:
partial class MyService : IMyService { partial void MyFirstFunction(); // Ok private partial void MySecondFunction(); // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers private partial void MyThirdFunction(); // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers private partial object MyFourthFunction(); // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers + CS0766 Partial methods must have a void return type private partial object MyFifthFunction(); // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers + CS0766 Partial methods must have a void return type private partial void MySixthFunction(out int result); // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers + CS0752 A partial method cannot have out parameters public partial void MySeventhFunction(); // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers }
partial class MyService { private partial void MyThirdFunction() { } // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers private partial object MyFifthFunction() { return new { }; } // CS0750 A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers + CS0766 Partial methods must have a void return type }
public interface IMyService { void MySeventhFunction(); }
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:
partial class MyService : IMyService { partial void MyFirstFunction(); // Ok private partial void MySecondFunction(); // CS8795 Partial method must have an implementation part because it has accessibility modifiers private partial void MyThirdFunction(); // Ok private partial object MyFourthFunction(); // CS8795 Partial method must have an implementation part because it has accessibility modifiers private partial object MyFifthFunction(); // Ok private partial void MySixthFunction(out int result); // CS8795 Partial method must have an implementation part because it has accessibility modifiers public partial void MySeventhFunction(); // Ok }
partial class MyService { private partial void MyThirdFunction() { } // Ok private partial object MyFifthFunction() { return new { }; } // Ok public partial void MySeventhFunction() { } // Ok }
public interface IMyService { void MySeventhFunction(); }
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 notreturn 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 ? 
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