Generated the JSON contracts of providers, the runtime, and services at compile time (#1150)

app/Tests/Serialization/JsonDefaultsTests.cs

@@ -7,11 +7,12 @@ namespace AIStudio.Tests.Serialization;
 /// Checks that a property missing in the JSON keeps the value a new object starts with.
 /// </summary>
 /// <remarks>
-/// Files written by an older version of AI Studio lack every property added since. Reading them has to give
-/// those properties their defaults, as a new object has them, e.g., a sub-object of the settings instead of
-/// null. Source generation in .NET 10 breaks this for init-only properties: it sets them all while creating
-/// the object, and a property missing in the JSON gets the default of its type instead (dotnet/runtime#84484).
-/// Hence, the types AI Studio stores use set instead of init.<br/><br/>
+/// Files written by an older version of AI Studio lack every property added since, and an answer of a
+/// provider, of the runtime, or of a service leaves out whatever it has nothing to say about. Reading them has
+/// to give those properties their defaults, as a new object has them, e.g., a sub-object of the settings
+/// instead of null. Source generation in .NET 10 breaks this for init-only properties: it sets them all while
+/// creating the object, and a property missing in the JSON gets the default of its type instead
+/// (dotnet/runtime#84484). Hence, the types AI Studio reads use set instead of init.<br/><br/>
 ///
 /// A new object is created the way reflection creates it: with its parameterless constructor, or, for a
 /// struct, as the default of the struct. Source generation passes the values of a positional record struct
@@ -21,6 +22,9 @@ namespace AIStudio.Tests.Serialization;
 public sealed class JsonDefaultsTests
 {
     [TestCaseSource(typeof(JsonTestRoots), nameof(JsonTestRoots.Stored))]
+    [TestCaseSource(typeof(JsonTestRoots), nameof(JsonTestRoots.ProviderAnswers))]
+    [TestCaseSource(typeof(JsonTestRoots), nameof(JsonTestRoots.RuntimeAnswers))]
+    [TestCaseSource(typeof(JsonTestRoots), nameof(JsonTestRoots.ServiceAnswers))]
     public void AnEmptyObjectGetsTheDefaultsOfANewOne(JsonTestRoot root)
     {
         var baseline = JsonReflectionBaseline.Of(root.Options);
@@ -60,7 +64,14 @@ public sealed class JsonDefaultsTests
         Assert.That(failures, Is.Empty);
     }
 
-    private static string ValueAsJson(JsonPropertyInfo property, object instance, JsonSerializerOptions options) => JsonSerializer.Serialize(property.Get!(instance), property.PropertyType, options);
+    private static string ValueAsJson(JsonPropertyInfo property, object instance, JsonSerializerOptions options) => AsJson(property.Get!(instance), property.PropertyType, options);
 
-    private static string DefaultAsJson(Type type, JsonSerializerOptions options) => type.IsValueType ? JsonSerializer.Serialize(Activator.CreateInstance(type), type, options) : "null";
+    private static string DefaultAsJson(Type type, JsonSerializerOptions options) => type.IsValueType ? AsJson(Activator.CreateInstance(type), type, options) : "null";
+
+    private static string AsJson(object? value, Type type, JsonSerializerOptions options) => value switch
+    {
+        // The default of a JsonElement holds no JSON at all, so it cannot be written:
+        JsonElement { ValueKind: JsonValueKind.Undefined } => "no JSON",
+        _ => JsonSerializer.Serialize(value, type, options),
+    };
 }