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Editorial Team
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Asked: 2026-06-06T04:19:43+00:00

I am developing an internal class that implements an internal interface. Can anyone explain

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I am developing an internal class that implements an internal interface.
Can anyone explain why I cannot declare my method as internal, why I am getting the following error: “cannot implement an interface member because it is not public”.

I know that I have to declare the method as public, but it makes absolutely no sense to me.

What is the point of declaring a method public if both the interface and the class are internal?
Is it not misleading?

I have read a related question on this site. It is not an exact duplicate, because my class is internal.

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1 Answer

  1. Editorial Team

    Simply put: because that’s the way the language designers designed it. Even in internal interfaces, the methods are implicitly public. It does make things simple, but it’s a pain in other ways.

    If you want a public class where you want to “hide” the use of an internal interface, you could use explicit interface implementation – although that has other drawbacks.

    Of course, if your class is internal then it doesn’t matter that the methods are public anyway – other assemblies aren’t going to be able to call the methods because they can’t see the type.

    I definitely agree that C# (or .NET in general) hasn’t been designed as carefully as it might be around internal interfaces.

    In terms of exactly why you’re getting an error message – section 13.4.4 of the C# 4 spec (interface mapping) is the reason. Implementations are only found for nonstatic public members and explicit interface member implementations – and if there are any unimplemented members in the interface, an error occurs.

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