Add discriminator-based polymorphism

Author: xoofx

src/Tomlyn.Tests/NewApiPolymorphismTests.cs

@@ -0,0 +1,133 @@
+using System.Text.Json.Serialization;
+using NUnit.Framework;
+using Tomlyn.Serialization;
+
+namespace Tomlyn.Tests;
+
+public class NewApiPolymorphismTests
+{
+    [TomlPolymorphic(TypeDiscriminatorPropertyName = "kind")]
+    [TomlDerivedType(typeof(Cat), "cat")]
+    [TomlDerivedType(typeof(Dog), "dog")]
+    private abstract class Animal
+    {
+        public string? Name { get; set; }
+    }
+
+    private sealed class Cat : Animal
+    {
+        public int Lives { get; set; }
+    }
+
+    private sealed class Dog : Animal
+    {
+        public bool GoodBoy { get; set; }
+    }
+
+    [Test]
+    public void Serialize_Polymorphic_WritesDiscriminator()
+    {
+        Animal value = new Cat { Name = "Nyan", Lives = 9 };
+        var toml = TomlSerializer.Serialize(value);
+
+        Assert.That(toml, Does.Contain("kind"));
+        Assert.That(toml, Does.Contain("cat"));
+        Assert.That(toml, Does.Contain("Name"));
+        Assert.That(toml, Does.Contain("Nyan"));
+        Assert.That(toml, Does.Contain("Lives"));
+        Assert.That(toml, Does.Contain("9"));
+    }
+
+    [Test]
+    public void Deserialize_Polymorphic_UsesDiscriminatorWhenNotFirst()
+    {
+        var toml =
+            """
+            kind = "cat"
+            Name = "Nyan"
+            Lives = 9
+            """;
+
+        var result = TomlSerializer.Deserialize<Animal>(toml);
+
+        Assert.That(result, Is.TypeOf<Cat>());
+        var cat = (Cat)result!;
+        Assert.That(cat.Name, Is.EqualTo("Nyan"));
+        Assert.That(cat.Lives, Is.EqualTo(9));
+    }
+
+    [Test]
+    public void Deserialize_Polymorphic_UnknownDiscriminator_FailsByDefault()
+    {
+        var toml =
+            """
+            kind = "fox"
+            Name = "X"
+            """;
+
+        Assert.Throws<TomlException>(() => TomlSerializer.Deserialize<Animal>(toml));
+    }
+
+    [TomlPolymorphic(TypeDiscriminatorPropertyName = "kind")]
+    [TomlDerivedType(typeof(DerivedPet), "derived")]
+    private class Pet
+    {
+        public string? Name { get; set; }
+    }
+
+    private sealed class DerivedPet : Pet
+    {
+        public int Age { get; set; }
+    }
+
+    [Test]
+    public void Deserialize_Polymorphic_UnknownDiscriminator_CanFallbackToBase()
+    {
+        var toml =
+            """
+            kind = "unknown"
+            Name = "Base"
+            extra = 123
+            """;
+
+        var options = new TomlSerializerOptions
+        {
+            PolymorphismOptions = new TomlPolymorphismOptions
+            {
+                TypeDiscriminatorPropertyName = "kind",
+                UnknownDerivedTypeHandling = TomlUnknownDerivedTypeHandling.FallBackToBaseType,
+            },
+        };
+
+        var result = TomlSerializer.Deserialize<Pet>(toml, options);
+        Assert.That(result, Is.TypeOf<Pet>());
+        Assert.That(result!.Name, Is.EqualTo("Base"));
+    }
+
+    [JsonPolymorphic(TypeDiscriminatorPropertyName = "kind")]
+    [JsonDerivedType(typeof(JsonCat), "cat")]
+    private abstract class JsonAnimal
+    {
+        public string? Name { get; set; }
+    }
+
+    private sealed class JsonCat : JsonAnimal
+    {
+        public int Lives { get; set; }
+    }
+
+    [Test]
+    public void Deserialize_Polymorphic_RespectsSystemTextJsonAttributes()
+    {
+        var toml =
+            """
+            kind = "cat"
+            Name = "Ada"
+            Lives = 9
+            """;
+
+        var result = TomlSerializer.Deserialize<JsonAnimal>(toml);
+        Assert.That(result, Is.TypeOf<JsonCat>());
+        Assert.That(((JsonCat)result!).Lives, Is.EqualTo(9));
+    }
+}