MsgPack012 Inaccessible data type

Serializable data types must have at least internal accessibility so that a source generated formatter elsewhere in the compilation can access it. C# defaults to internal visibility except for nested types, which receive private visibility by default.

Examples of patterns that are flagged by this analyzer

class Outer {
    [MessagePackObject]
    /*private*/ class DataType { }
}

Typical fix

Add the internal or public modifier to the nested data type:

class Outer {
    [MessagePackObject]
    internal class DataType { }
}