MsgPack010 Inaccessible Formatter

Formatters must be declared with internal or public visibility so that the source generated resolver can access them.

This tends to happen when a formatter is declared as a nested type, where C# defaults to private visibility.

Examples of patterns that are flagged by this analyzer

class Outer {
    /*private*/ class CustomBFormatter : IMessagePackFormatter<B> { // MsgPack010
        void Serialize(ref MessagePackWriter writer, B value, MessagePackSerializerOptions options)
            => throw new NotImplementedException();

        B Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
            => throw new NotImplementedException();
    }
}

Typical fix

Add the internal or public modifier to the nested formatter:

class Outer {
    internal class CustomBFormatter : IMessagePackFormatter<B> {
        void Serialize(ref MessagePackWriter writer, B value, MessagePackSerializerOptions options)
            => throw new NotImplementedException();

        B Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
            => throw new NotImplementedException();
    }
}