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This code:

public class Class1 { private Class2 _class2; private void Something() { Class2 class2; } } file class Class2 { }

produces compiler error CS9051 on the member _class2 but not on the local variable class2 inside the method:

File-local type 'Class2' cannot be used in a member signature in non-file-local type 'Class1'.

Clicking on the link provided with CS9051 opens this page, which at the time of this writing states Sorry, we don't have specifics on this C# error. I couldn't find anything else on the web which explains why this is the case. Can someone please explain why I can use Class2 inside of a method or property within Class1 but not as a private class member? It seems like this reduces the usefulness of an otherwise-useful feature (the file access modifier).

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  • Just checking: Class1 isn't a partial class or anything like that? Commented Mar 2, 2024 at 15:50
  • I presume that having the variable as a private member makes it accessible to the outside world via reflection, whereas a variable inside a method cannot escape. Commented Mar 2, 2024 at 15:54
  • 1
    "Why" is not a good question. Please clarify what kind of answer you are expecting. Commented Mar 2, 2024 at 15:55
  • @Sweeper Something like "the reason for this is ______" Commented Mar 2, 2024 at 15:59
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    @Sweeper Can you stop being pedantic? If you want, you go ahead and edit my question. I think it's pretty clear. Commented Mar 2, 2024 at 16:03

2 Answers 2

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The proposal says Only allow signature usage in members of file-local types. There is also an example showing your use case.

A workaround is to type the fields as object and to cast the field when using it:

public class Class1 { private object _class2 = new Class2(); private void Something() { Class2 class2 = (Class2)_class2; } } file class Class2 { }
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2 Comments

Could you rephrase a bit this answer? I am not sure to understand what "signature" means here. I know about "signature" in the context of a method, but it does not look like this is the right context. In other words, even after reading the spec, I don't understand why a private class member could not be file-scoped.
They are referring to signatures of any member type (even for private members), which includes your private field (at least that's how I understand it, even if it is not clear to me why this affects private members).
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Permit a file modifier on top-level type declarations. The type only exists in the file where it is declared.

https://learn.microsoft.com/En-Us/dotnet/csharp/language-reference/proposals/csharp-11.0/file-local-types

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Both the classes in the example code are in the same file.

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