Apply UnknownDerivedTypeHandling.Fail to tag-based polymorphism

src/SharpYaml.SourceGenerator/YamlSerializerContextGenerator.cs

@@ -3316,6 +3316,19 @@ private static void EmitReadValue(
                     builder.Append("                return (").Append(typeName).Append(")ReadValue").Append(derivedIndex).AppendLine("(bufferedReader)!;");
                     builder.AppendLine("            }");
                 }
+
+                // When tag is present but unrecognized, try default type before failing
+                if (polymorphism.DefaultDerivedType is not null && indexByType.TryGetValue(polymorphism.DefaultDerivedType, out var defaultIndexForUnknownTag))
+                {
+                    builder.Append("            return (").Append(typeName).Append(")ReadValue").Append(defaultIndexForUnknownTag).AppendLine("(bufferedReader)!;");
+                }
+                else
+                {
+                    builder.AppendLine("            if (unknownDerivedTypeHandling == global::SharpYaml.YamlUnknownDerivedTypeHandling.Fail)");
+                    builder.AppendLine("            {");
+                    builder.Append("                throw global::SharpYaml.Serialization.YamlThrowHelper.ThrowUnknownTypeTag(bufferedReader, rootTag, typeof(").Append(typeName).AppendLine("));");
+                    builder.AppendLine("            }");
+                }
                 builder.AppendLine("        }");
 
                 // Fallback: use default derived type if available

src/SharpYaml.Tests/Serialization/YamlUnknownTagHandlingTests.cs

@@ -0,0 +1,275 @@
+#nullable enable
+
+using System.Collections.Generic;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using SharpYaml.Serialization;
+
+namespace SharpYaml.Tests.Serialization;
+
+[TestClass]
+public class YamlUnknownTagHandlingTests
+{
+    // ---- Model types ----
+
+    [YamlPolymorphic(DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag)]
+    [YamlDerivedType(typeof(AttributeDog), Tag = "!dog")]
+    [YamlDerivedType(typeof(AttributeCat), Tag = "!cat")]
+    private class AttributeAnimal
+    {
+        public string Name { get; set; } = string.Empty;
+    }
+
+    private sealed class AttributeDog : AttributeAnimal
+    {
+        public int BarkVolume { get; set; }
+    }
+
+    private sealed class AttributeCat : AttributeAnimal
+    {
+        public bool Indoor { get; set; }
+    }
+
+    [YamlPolymorphic(DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag, UnknownDerivedTypeHandling = YamlUnknownDerivedTypeHandling.FallBackToBase)]
+    [YamlDerivedType(typeof(FallbackDog), Tag = "!dog")]
+    private class FallbackAnimal
+    {
+        public string Name { get; set; } = string.Empty;
+    }
+
+    private sealed class FallbackDog : FallbackAnimal
+    {
+        public int BarkVolume { get; set; }
+    }
+
+    private abstract class RuntimeAnimal
+    {
+        public string Name { get; set; } = string.Empty;
+    }
+
+    private sealed class RuntimeDog : RuntimeAnimal
+    {
+        public int BarkVolume { get; set; }
+    }
+
+    private sealed class RuntimeCat : RuntimeAnimal
+    {
+        public bool Indoor { get; set; }
+    }
+
+    // ---- Attribute-based: unknown tag with default (Fail) handling ----
+
+    [TestMethod]
+    public void UnknownTagFailsByDefaultWithAttributes()
+    {
+        var yaml = "!lizard\nName: Gecko\n";
+        var ex = Assert.Throws<YamlException>(
+            () => YamlSerializer.Deserialize<AttributeAnimal>(yaml));
+        StringAssert.Contains(ex.Message, "!lizard");
+    }
+
+    [TestMethod]
+    public void KnownTagWorksWithAttributes()
+    {
+        var yaml = "!dog\nName: Rex\nBarkVolume: 5\n";
+        var value = YamlSerializer.Deserialize<AttributeAnimal>(yaml);
+
+        Assert.IsNotNull(value);
+        Assert.IsInstanceOfType<AttributeDog>(value);
+        Assert.AreEqual("Rex", value.Name);
+        Assert.AreEqual(5, ((AttributeDog)value).BarkVolume);
+    }
+
+    [TestMethod]
+    public void NoTagDeserializesToBaseTypeWithAttributes()
+    {
+        var yaml = "Name: Plain\n";
+        var value = YamlSerializer.Deserialize<AttributeAnimal>(yaml);
+
+        Assert.IsNotNull(value);
+        Assert.IsInstanceOfType<AttributeAnimal>(value);
+        Assert.AreEqual("Plain", value.Name);
+    }
+
+    // ---- Attribute-based: FallBackToBase ----
+
+    [TestMethod]
+    public void UnknownTagFallsBackToBaseWhenConfigured()
+    {
+        var yaml = "!lizard\nName: Gecko\n";
+        var value = YamlSerializer.Deserialize<FallbackAnimal>(yaml);
+
+        Assert.IsNotNull(value);
+        Assert.IsInstanceOfType<FallbackAnimal>(value);
+        Assert.AreEqual("Gecko", value.Name);
+    }
+
+    // ---- Options-level: Fail ----
+
+    [TestMethod]
+    public void UnknownTagFailsWithOptionsLevelFail()
+    {
+        var options = new YamlSerializerOptions
+        {
+            PolymorphismOptions = new YamlPolymorphismOptions
+            {
+                DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag,
+                UnknownDerivedTypeHandling = YamlUnknownDerivedTypeHandling.Fail,
+                DerivedTypeMappings =
+                {
+                    [typeof(RuntimeAnimal)] = new List<YamlDerivedType>
+                    {
+                        new YamlDerivedType(typeof(RuntimeDog), "dog") { Tag = "!dog" },
+                        new YamlDerivedType(typeof(RuntimeCat), "cat") { Tag = "!cat" },
+                    }
+                }
+            }
+        };
+
+        var yaml = "!parrot\nName: Polly\n";
+        var ex = Assert.Throws<YamlException>(
+            () => YamlSerializer.Deserialize<RuntimeAnimal>(yaml, options));
+        StringAssert.Contains(ex.Message, "!parrot");
+    }
+
+    [TestMethod]
+    public void KnownTagWorksWithRuntime()
+    {
+        var options = new YamlSerializerOptions
+        {
+            PolymorphismOptions = new YamlPolymorphismOptions
+            {
+                DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag,
+                DerivedTypeMappings =
+                {
+                    [typeof(RuntimeAnimal)] = new List<YamlDerivedType>
+                    {
+                        new YamlDerivedType(typeof(RuntimeDog), "dog") { Tag = "!dog" },
+                    }
+                }
+            }
+        };
+
+        var yaml = "!dog\nName: Rex\nBarkVolume: 3\n";
+        var value = YamlSerializer.Deserialize<RuntimeAnimal>(yaml, options);
+
+        Assert.IsNotNull(value);
+        Assert.IsInstanceOfType<RuntimeDog>(value);
+        Assert.AreEqual("Rex", value.Name);
+        Assert.AreEqual(3, ((RuntimeDog)value).BarkVolume);
+    }
+
+    // ---- Options-level: FallBackToBase ----
+
+    private class ConcreteAnimal
+    {
+        public string Name { get; set; } = string.Empty;
+    }
+
+    private sealed class ConcreteDog : ConcreteAnimal
+    {
+        public int BarkVolume { get; set; }
+    }
+
+    [TestMethod]
+    public void UnknownTagFallsBackToBaseWithOptions()
+    {
+        var options = new YamlSerializerOptions
+        {
+            PolymorphismOptions = new YamlPolymorphismOptions
+            {
+                DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag,
+                UnknownDerivedTypeHandling = YamlUnknownDerivedTypeHandling.FallBackToBase,
+                DerivedTypeMappings =
+                {
+                    [typeof(ConcreteAnimal)] = new List<YamlDerivedType>
+                    {
+                        new YamlDerivedType(typeof(ConcreteDog), "dog") { Tag = "!dog" },
+                    }
+                }
+            }
+        };
+
+        var yaml = "!parrot\nName: Polly\n";
+        var value = YamlSerializer.Deserialize<ConcreteAnimal>(yaml, options);
+
+        Assert.IsNotNull(value);
+        Assert.IsInstanceOfType<ConcreteAnimal>(value);
+        Assert.AreEqual("Polly", value.Name);
+    }
+
+    // ---- Attribute-level override takes precedence over options ----
+
+    [TestMethod]
+    public void AttributeUnknownHandlingOverridesOptions()
+    {
+        // FallbackAnimal has FallBackToBase in attribute; options say Fail — attribute wins
+        var options = new YamlSerializerOptions
+        {
+            PolymorphismOptions = new YamlPolymorphismOptions
+            {
+                UnknownDerivedTypeHandling = YamlUnknownDerivedTypeHandling.Fail,
+            }
+        };
+
+        var yaml = "!lizard\nName: Gecko\n";
+        var value = YamlSerializer.Deserialize<FallbackAnimal>(yaml, options);
+
+        Assert.IsNotNull(value);
+        Assert.IsInstanceOfType<FallbackAnimal>(value);
+        Assert.AreEqual("Gecko", value.Name);
+    }
+
+    // ---- Dictionary of polymorphic values with unknown tags ----
+
+    [TestMethod]
+    public void UnknownTagInDictionaryValueFails()
+    {
+        var options = new YamlSerializerOptions
+        {
+            PolymorphismOptions = new YamlPolymorphismOptions
+            {
+                DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag,
+                UnknownDerivedTypeHandling = YamlUnknownDerivedTypeHandling.Fail,
+                DerivedTypeMappings =
+                {
+                    [typeof(RuntimeAnimal)] = new List<YamlDerivedType>
+                    {
+                        new YamlDerivedType(typeof(RuntimeDog), "dog") { Tag = "!dog" },
+                    }
+                }
+            }
+        };
+
+        var yaml = "rex: !dog\n  Name: Rex\n  BarkVolume: 3\npolly: !parrot\n  Name: Polly\n";
+        Assert.Throws<YamlException>(
+            () => YamlSerializer.Deserialize<Dictionary<string, RuntimeAnimal>>(yaml, options));
+    }
+
+    // ---- Tag-only entries (no discriminator) with unknown tags ----
+
+    [TestMethod]
+    public void UnknownTagFailsWithTagOnlyEntries()
+    {
+        var options = new YamlSerializerOptions
+        {
+            PolymorphismOptions = new YamlPolymorphismOptions
+            {
+                DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag,
+                UnknownDerivedTypeHandling = YamlUnknownDerivedTypeHandling.Fail,
+                DerivedTypeMappings =
+                {
+                    [typeof(RuntimeAnimal)] = new List<YamlDerivedType>
+                    {
+                        new YamlDerivedType(typeof(RuntimeDog)) { Tag = "!dog" },
+                        new YamlDerivedType(typeof(RuntimeCat)) { Tag = "!cat" },
+                    }
+                }
+            }
+        };
+
+        var yaml = "!fish\nName: Nemo\n";
+        var ex = Assert.Throws<YamlException>(
+            () => YamlSerializer.Deserialize<RuntimeAnimal>(yaml, options));
+        StringAssert.Contains(ex.Message, "!fish");
+    }
+}

src/SharpYaml/Serialization/Converters/YamlObjectConverter.cs

@@ -1350,6 +1350,14 @@ private void WriteObjectCore(YamlWriter writer, object value, Contract contract)
             {
                 targetType = derivedFromTag;
             }
+            else if (polymorphism.DefaultDerivedType is not null)
+            {
+                targetType = polymorphism.DefaultDerivedType;
+            }
+            else if (polymorphism.UnknownDerivedTypeHandling == YamlUnknownDerivedTypeHandling.Fail)
+            {
+                throw YamlThrowHelper.ThrowUnknownTypeTag(reader, rootTag, typeof(T));
+            }
         }
 
         targetType ??= polymorphism.DefaultDerivedType ?? typeof(T);

src/SharpYaml/Serialization/YamlThrowHelper.cs

@@ -52,6 +52,10 @@ public static YamlException ThrowDuplicateMappingKey(YamlReader reader, string k
     public static YamlException ThrowUnknownTypeDiscriminator(YamlReader reader, string? discriminatorValue, Type baseType)
         => new(reader.SourceName, reader.Start, reader.End, $"Unknown type discriminator '{discriminatorValue}' for '{baseType}'.");
 
+    /// <summary>Throws an exception for unknown Type Tag.</summary>
+    public static YamlException ThrowUnknownTypeTag(YamlReader reader, string? tag, Type baseType)
+        => new(reader.SourceName, reader.Start, reader.End, $"Unknown type tag '{tag}' for '{baseType}'.");
+
     /// <summary>Throws an exception for abstract Type Without Discriminator.</summary>
     public static YamlException ThrowAbstractTypeWithoutDiscriminator(YamlReader reader, Type type)
         => new(reader.SourceName, reader.Start, reader.End, $"Cannot deserialize abstract type '{type}' without a known derived type discriminator.");