NBMsgPack010: [KnownSubType] alias must be unique

@Nerdbank.MessagePack.KnownSubTypeAttribute should specify an alias that is unique within the scope of the type it is applied to.

Learn more about subtype unions.

Example violations

[KnownSubType<DerivedType1>(1)]
[KnownSubType<DerivedType2>(1)] // Reused an alias
class BaseType
{
}

class DerivedType1 : BaseType
{
}

class DerivedType2 : BaseType
{
}

Resolution

Change the alias to one that has not yet been used.

[KnownSubType<DerivedType1>(1)]
[KnownSubType<DerivedType2>(2)]
class BaseType
{
}

class DerivedType1 : BaseType
{
}

class DerivedType2 : BaseType
{
}

Additional examples

Note that across types, the alias does not need to be unique. The following is perfectly valid:

[KnownSubType<DerivedFromBaseType>(1)]
class BaseType
{
}

[KnownSubType<DerivedFromAnotherType>(1)]
class AnotherType
{
}