MsgPack015 MessagePackObjectAttribute.AllowPrivate should be set

When a [MessagePackObject]-attributed type cannot be serialized without access to non-public members, the AllowPrivate property should be set on the MessagePackObjectAttribute so that:

• Dynamically generated formatters can serialize the type, even if DynamicObjectResolver is used instead of DynamicObjectResolverAllowPrivate.
• Analyzers can help ensure proper annotation of non-public members.

Examples of patterns that are flagged by this analyzer

Internal members

[MessagePackObject]
public class MyData {
    [Key(0)]
    internal int Foo { get; set; }
}

Internal type

[MessagePackObject]
internal class MyData {
    [Key(0)]
    public int Foo { get; set; }
}

Typical fix

Simply add the AllowPrivate = true syntax to the [MessagePackObject] attribute:

[MessagePackObject(AllowPrivate = true)]
internal class MyData {
    [Key(0)]
    internal int Foo { get; set; }
}

An automated code fix is offered for this diagnostic.