Motivation

When using YamlDerivedType(Type) or YamlDerivedTypeAttribute(Type) with only a Tag (no discriminator), the constructor doc comment stated the entry would be "the default when no discriminator matches." This was misleading — the actual behavior correctly treats tag-only entries as tag dispatch participants only, not as the default derived type.

Scenario

DerivedTypeMappings =
{
    [typeof(PullMonitorConfiguration)] =
    [
        new YamlDerivedType(typeof(CheckInMonitorConfiguration)) { Tag = \"!checkin\" },
        new YamlDerivedType(typeof(DnsMonitorConfiguration)) { Tag = \"!dns\" },
    ]
}

With this configuration, an untagged YAML mapping should deserialize as the base type PullMonitorConfiguration, not as CheckInMonitorConfiguration.

Changes

1. Clarified doc comments on both YamlDerivedType(Type) and YamlDerivedTypeAttribute(Type) constructors to explicitly document that:

   • Default is set only when both Discriminator and Tag are null
   • When Tag is set, the entry participates in tag-based dispatch only

2. Added 10 comprehensive tests covering:

   • Attribute-based tag-only entries: untagged values deserialize as base type
   • Runtime tag-only entries (DerivedTypeMappings): untagged values deserialize as base type
   • Dictionary with mixed tagged and untagged values
   • Explicit default type coexisting with tag-only entries
   • Roundtrip serialization for both attribute and runtime paths