Add JSON source-gen mode that emits serialization logic

[layomia]

Contributes to #51945.
Contributes to #52279.

Users kick off source generation by providing a partial, derived context class; and indicate serializable types and run-time options (optional):

namespace System.Text.Json.SourceGeneration.Tests
{
    [JsonSerializerOptions(
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault,
        IgnoreReadOnlyProperties = true,
        IgnoreRuntimeCustomConverters = true,
        NamingPolicy = JsonKnownNamingPolicy.BuiltInCamelCase)]
    [JsonSerializable(typeof(JsonMessage), GenerationMode = JsonSourceGenerationMode.Serialization)]
    internal partial class JsonContext : JsonSerializerContext
    {
    }

    public class JsonMessage
    {
        public string Message { get; set; }
        public int Length => Message?.Length ?? 0; // Read-only property
    }
}

Generated code (click to view)

// <auto-generated/>

namespace System.Text.Json.SourceGeneration.Tests
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Text.Json.SourceGeneration", "6.0.0.0")]
    internal partial class JsonContext : global::System.Text.Json.Serialization.JsonSerializerContext
    {
        private static global::System.Text.Json.JsonSerializerOptions s_defaultOptions { get; } = new global::System.Text.Json.JsonSerializerOptions()
            {
                DefaultIgnoreCondition = global::System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault,
                IgnoreReadOnlyFields = false,
                IgnoreReadOnlyProperties = true,
                IncludeFields = false,
                WriteIndented = false,
                PropertyNamingPolicy = global::System.Text.Json.JsonNamingPolicy.CamelCase
            };
    
        private static global::System.Text.Json.SourceGeneration.Tests.JsonContext s_defaultContext;
        public static global::System.Text.Json.SourceGeneration.Tests.JsonContext Default => s_defaultContext ??= new global::System.Text.Json.SourceGeneration.Tests.JsonContext(new global::System.Text.Json.JsonSerializerOptions(s_defaultOptions));
    
        private static global::System.Text.Json.JsonEncodedText messagePropName = global::System.Text.Json.JsonEncodedText.Encode("message");
        private static global::System.Text.Json.JsonEncodedText lengthPropName = global::System.Text.Json.JsonEncodedText.Encode("length");
    
        public JsonContext() : base(null, s_defaultOptions)
        {
        }
    
        public JsonContext(global::System.Text.Json.JsonSerializerOptions options) : base(options, s_defaultOptions)
        {
        }
    
        public override global::System.Text.Json.Serialization.Metadata.JsonTypeInfo GetTypeInfo(global::System.Type type)
        {
            if (type == typeof(global::System.Text.Json.SourceGeneration.Tests.JsonMessage))
            {
                return this.JsonMessage;
            }
    
            return null!;
        }

        private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage> _JsonMessage;
        public global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage> JsonMessage
        {
            get
            {
                if (_JsonMessage == null)
                {
                    global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage> objectInfo = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreateObjectInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage>();
                    _JsonMessage = objectInfo;
    
                    global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.InitializeObjectInfo(
                        objectInfo,
                        Options,
                        createObjectFunc: static () => new global::System.Text.Json.SourceGeneration.Tests.JsonMessage(),
                        propInitFunc: null,
                        default,
                        serializeFunc: JsonMessageSerialize);
                }
    
                return _JsonMessage;
            }
        }
    
        private static void JsonMessageSerialize(global::System.Text.Json.Utf8JsonWriter writer, global::System.Text.Json.SourceGeneration.Tests.JsonMessage value)
        {
            if (value == null)
            {
                writer.WriteNullValue();
                return;
            }
    
            writer.WriteStartObject();

            if (value.Message != null)
            {
                writer.WriteString(messagePropName, value.Message);
            }
    
            writer.WriteEndObject();
        }
    }
}

To use generated serialization code:

Interacting with generated func directly:

using MemoryStream ms = new();
Utf8JsonWriter writer = new(ms);
JsonContext.Default.JsonMessage.Serialize!(writer, new JsonMessage { Message = "Hello, world!" });
writer.Flush();

Interacting via JsonSerializer:

JsonSerializer.Serialize(new JsonMessage { Message = "Hello, world!" }, JsonContext.Default.JsonMessage);

or

JsonSerializer.Serialize(new JsonMessage { Message = "Hello, world!" }, typeof(JsonMessage), JsonContext.Default);

FYI @pranavkm @SteveSandersonMS @terrajobst @ericstj @jkotas @stephentoub @davidfowl @SamMonoRT @CoffeeFlux

[ghost]

Tagging subscribers to this area: @eiriktsarpalis, @layomia
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Contributes to #51945.

Users kick off source generation by providing a partial, derived context class; and indicate serializable types and run-time options (optional):

namespace System.Text.Json.SourceGeneration.Tests
{
    [JsonSerializerOptions(
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault,
        IgnoreReadOnlyProperties = true,
        IgnoreRuntimeCustomConverters = true,
        NamingPolicy = JsonKnownNamingPolicy.BuiltInCamelCase)]
    [JsonSerializable(typeof(JsonMessage), GenerationMode = JsonSourceGenerationMode.Serialization)]
    internal partial class JsonContext : JsonSerializerContext
    {
    }

    public class JsonMessage
    {
        public string Message { get; set; }
        public int Length => Message?.Length ?? 0; // Read-only property
    }
}

Generated code (click to view)

// <auto-generated/>

namespace System.Text.Json.SourceGeneration.Tests
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Text.Json.SourceGeneration", "6.0.0.0")]
    internal partial class JsonContext : global::System.Text.Json.Serialization.JsonSerializerContext
    {
        private static global::System.Text.Json.JsonSerializerOptions s_defaultOptions { get; } = new global::System.Text.Json.JsonSerializerOptions()
            {
                DefaultIgnoreCondition = global::System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault,
                IgnoreReadOnlyFields = false,
                IgnoreReadOnlyProperties = true,
                IncludeFields = false,
                WriteIndented = false,
                PropertyNamingPolicy = global::System.Text.Json.JsonNamingPolicy.CamelCase
            };
    
        private static global::System.Text.Json.SourceGeneration.Tests.JsonContext s_defaultContext;
        public static global::System.Text.Json.SourceGeneration.Tests.JsonContext Default => s_defaultContext ??= new global::System.Text.Json.SourceGeneration.Tests.JsonContext(new global::System.Text.Json.JsonSerializerOptions(s_defaultOptions));
    
        private static global::System.Text.Json.JsonEncodedText messagePropName = global::System.Text.Json.JsonEncodedText.Encode("message");
        private static global::System.Text.Json.JsonEncodedText lengthPropName = global::System.Text.Json.JsonEncodedText.Encode("length");
    
        public JsonContext() : base(null, s_defaultOptions)
        {
        }
    
        public JsonContext(global::System.Text.Json.JsonSerializerOptions options) : base(options, s_defaultOptions)
        {
        }
    
        public override global::System.Text.Json.Serialization.Metadata.JsonTypeInfo GetTypeInfo(global::System.Type type)
        {
            if (type == typeof(global::System.Text.Json.SourceGeneration.Tests.JsonMessage))
            {
                return this.JsonMessage;
            }
    
            return null!;
        }

        private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage> _JsonMessage;
        public global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage> JsonMessage
        {
            get
            {
                if (_JsonMessage == null)
                {
                    global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage> objectInfo = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreateObjectInfo<global::System.Text.Json.SourceGeneration.Tests.JsonMessage>();
                    _JsonMessage = objectInfo;
    
                    global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.InitializeObjectInfo(
                        objectInfo,
                        Options,
                        createObjectFunc: static () => new global::System.Text.Json.SourceGeneration.Tests.JsonMessage(),
                        propInitFunc: null,
                        default,
                        serializeFunc: JsonMessageSerialize);
                }
    
                return _JsonMessage;
            }
        }
    
        private static void JsonMessageSerialize(global::System.Text.Json.Utf8JsonWriter writer, global::System.Text.Json.SourceGeneration.Tests.JsonMessage value)
        {
            if (value == null)
            {
                writer.WriteNullValue();
                return;
            }
    
            writer.WriteStartObject();

            if (value.Message != null)
            {
                writer.WriteString(messagePropName, value.Message);
            }
    
            writer.WriteEndObject();
        }
    }
}

To use generated serialization code:

Interacting with generated func directly:

using MemoryStream ms = new();
Utf8JsonWriter writer = new(ms);
JsonContext.Default.JsonMessage.Serialize!(writer, new JsonMessage { Message = "Hello, world!" });
writer.Flush();

Interacting via JsonSerializer:

JsonSerializer.Serialize(new JsonMessage { Message = "Hello, world!" }, JsonContext.Default.JsonMessage);

FYI @pranavkm @SteveSandersonMS @terrajobst @ericstj @jkotas @stephentoub @davidfowl @SamMonoRT @CoffeeFlux

Author:	layomia
Assignees:	layomia
Labels:	area-System.Text.Json
Milestone:	6.0.0

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[steveharter]

Why use S.R.S.CollectionDataContractAttribute here?

[layomia]

I changed to this because, like JsonSerializableAttribute, it is applied to classes.

[layomia]

This test is modeling type aliasing. In this case, we have two attributes switching names. The test makes sure that our check for [JsonSerializable(Type)] guards against such tricks.

[steveharter]

I see. A comment here about that would be useful. Thanks

[ericstj]

Lgtm. Let’s get this in so we can get a preview s out. Any additional feedback can be addressed in follow up prs.

[ericstj]

All these PGO failures were caused by #53301, @AndyAyersMS and @BruceForstall have helped get this fixed, but we can't get rid of the badges without pushing new changes / resetting CI. Since we know this PR didn't introduce the PGO failures we can safely ignore those.

[layomia]

The Build Browser wasm Release AllSubsets_Mono failure is unrelated and might be fixed by #53280. cc @ericstj

[layomia]

/backport to release/6.0-preview5

[github-actions]

Started backporting to release/6.0-preview5: https://github.com/dotnet/runtime/actions/runs/881152471

[layomia]

These breaking changes have already hit customers & the APIs have changed further since then - dotnet/docs#26200.