Add generated JSON context for Chat model objects including additional properties

Author: kzu

src/AI/AdditionalPropertiesDictionaryConverter.cs

@@ -0,0 +1,73 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Microsoft.Extensions.AI;
+
+namespace Devlooped.Extensions.AI;
+
+/// <summary>
+/// A JSON converter for <see cref="AdditionalPropertiesDictionary"/> that handles serialization and deserialization 
+/// of additional properties so they are stored and retrieved as primitive types.
+/// </summary>
+partial class AdditionalPropertiesDictionaryConverter : JsonConverter<AdditionalPropertiesDictionary>
+{
+    public override AdditionalPropertiesDictionary Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType != JsonTokenType.StartObject)
+            throw new JsonException("Expected start of object.");
+
+        var dictionary = new AdditionalPropertiesDictionary();
+
+        while (reader.Read())
+        {
+            if (reader.TokenType == JsonTokenType.EndObject)
+                return dictionary;
+
+            if (reader.TokenType != JsonTokenType.PropertyName)
+                throw new JsonException("Expected property name.");
+
+            var key = reader.GetString()!;
+            reader.Read();
+
+            var value = JsonSerializer.Deserialize<object>(ref reader, options);
+            if (value is JsonElement element)
+                dictionary[key] = GetPrimitive(element);
+            else
+                dictionary[key] = value;
+        }
+
+        throw new JsonException("Unexpected end of JSON.");
+    }
+
+    public override void Write(Utf8JsonWriter writer, AdditionalPropertiesDictionary value, JsonSerializerOptions options)
+    {
+        writer.WriteStartObject();
+
+        foreach (var kvp in value.Where(x => x.Value is not null))
+        {
+            writer.WritePropertyName(kvp.Key);
+            JsonSerializer.Serialize(writer, kvp.Value, options);
+        }
+
+        writer.WriteEndObject();
+    }
+
+    // Helper to convert JsonElement to closest .NET primitive
+    static object? GetPrimitive(JsonElement element)
+    {
+        switch (element.ValueKind)
+        {
+            case JsonValueKind.String: return element.GetString();
+            case JsonValueKind.Number:
+                if (element.TryGetInt32(out var i)) return i;
+                if (element.TryGetInt64(out var l)) return l;
+                if (element.TryGetDouble(out var d)) return d;
+                return element.GetDecimal();
+            case JsonValueKind.True: return true;
+            case JsonValueKind.False: return false;
+            case JsonValueKind.Null: return null;
+            case JsonValueKind.Object: return element; // You can recurse here if needed
+            case JsonValueKind.Array: return element;  // Or parse as List<object?>
+            default: return element;
+        }
+    }
+}

src/AI/ChatJsonContext.cs

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+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Encodings.Web;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
+using Microsoft.Extensions.AI;
+
+namespace Devlooped.Extensions.AI;
+
+[JsonSourceGenerationOptions(JsonSerializerDefaults.Web,
+    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull | JsonIgnoreCondition.WhenWritingDefault,
+    UseStringEnumConverter = true,
+    UnmappedMemberHandling = JsonUnmappedMemberHandling.Skip,
+    PropertyNameCaseInsensitive = true,
+    PropertyNamingPolicy = JsonKnownNamingPolicy.SnakeCaseLower
+#if DEBUG
+    , WriteIndented = true
+#endif
+)]
+[JsonSerializable(typeof(Chat))]
+[JsonSerializable(typeof(ChatResponse))]
+[JsonSerializable(typeof(ChatMessage))]
+[JsonSerializable(typeof(AdditionalPropertiesDictionary))]
+public partial class ChatJsonContext : JsonSerializerContext
+{
+    static readonly Lazy<JsonSerializerOptions> options = new(CreateDefaultOptions);
+
+    /// <summary>
+    /// Provides a pre-configured instance of <see cref="JsonSerializerOptions"/> that aligns with the context's settings.
+    /// </summary>
+    public static JsonSerializerOptions DefaultOptions { get => options.Value; }
+
+    [UnconditionalSuppressMessage("AotAnalysis", "IL3050", Justification = "DefaultJsonTypeInfoResolver is only used when reflection-based serialization is enabled")]
+    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026", Justification = "DefaultJsonTypeInfoResolver is only used when reflection-based serialization is enabled")]
+    static JsonSerializerOptions CreateDefaultOptions()
+    {
+        JsonSerializerOptions options = new(Default.Options)
+        {
+            Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
+            WriteIndented = Debugger.IsAttached,
+        };
+
+        if (JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            // If reflection-based serialization is enabled by default, use it as a fallback for all other types.
+            // Also turn on string-based enum serialization for all unknown enums.
+            options.TypeInfoResolverChain.Add(new DefaultJsonTypeInfoResolver());
+            options.Converters.Add(new JsonStringEnumConverter());
+        }
+
+        // Make sure we deserialize AdditionalProperties values to their primitive types
+        options.Converters.Insert(0, new AdditionalPropertiesDictionaryConverter());
+
+        options.MakeReadOnly();
+        return options;
+    }
+}