System.Text.Json does not fill JsonExtensionData when a property with the same name already exists or has the JsonIgnoreAttribute

[softlion]

Description

The JsonExtensionData property does not contain the properties from the deserialized Json that have matching read only or JsonIgnore properties on the target object.

Reproduction Steps

When deserializing that json:

var json = @"{ ""value"": { ""content"": ""test"" } }";

into that object

    public class TestDefault
    {
        [JsonIgnore]
        public JsonElement Value => Values["value"];
        
        [JsonExtensionData]
        public Dictionary<string, JsonElement> Values { get; set; }
    }

with this code:

    [TestClass]
    public class TestNotionHtml
    {
        [TestMethod]
        public void TestJsonExtensionData_Inherited()
        {
            var json = @"{ ""value"": { ""content"": ""test"" } }";
            var r = JsonSerializer.Deserialize<TestDefault>(json, new(JsonSerializerDefaults.Web));
            Assert.IsNotNull(r);
            Assert.IsNotNull(r.Values);
            Assert.AreEqual(1, r.Values.Count);
            Assert.IsTrue(r.Values.ContainsKey("value"));
            Assert.IsNotNull(r.Value);
        }
  }

the Values dictionary is empty.

Expected behavior

Values should contains all properties that are not deserialized

Actual behavior

Values is empty

Regression?

No

Known Workarounds

None

Configuration

.net6

system.text.json 6.0.3

Other information

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

The JsonExtensionData property does not contain the properties from the deserialized Json that have matching read only or JsonIgnore properties on the target object.

Reproduction Steps

When deserializing that json:

var json = @"{ ""value"": { ""content"": ""test"" } }";

into that object

    public class TestDefault
    {
        [JsonIgnore]
        public JsonElement Value => Values["value"];
        
        [JsonExtensionData]
        public Dictionary<string, JsonElement> Values { get; set; }
    }

with this code:

    [TestClass]
    public class TestNotionHtml
    {
        [TestMethod]
        public void TestJsonExtensionData_Inherited()
        {
            var json = @"{ ""value"": { ""content"": ""test"" } }";
            var r = JsonSerializer.Deserialize<TestDefault>(json, new(JsonSerializerDefaults.Web));
            Assert.IsNotNull(r);
            Assert.IsNotNull(r.Values);
            Assert.AreEqual(1, r.Values.Count);
            Assert.IsTrue(r.Values.ContainsKey("value"));
            Assert.IsNotNull(r.Value);
        }
  }

the Values dictionary is empty.

Expected behavior

Values should contains all properties that are not deserialized

Actual behavior

Values is empty

Regression?

No

Known Workarounds

None

Configuration

.net6

system.text.json 6.0.3

Other information

No response

Author:	softlion
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[eiriktsarpalis]

I can reproduce, here's a minimal reproduction:

var json = @"{ ""Value"": true }";
var r = JsonSerializer.Deserialize<TestDefault>(json);
Console.WriteLine(r.ExtensionData?.Count ?? 0); // prints 0

public class TestDefault
{
    public bool Value { get; }

    [JsonExtensionData]
    public Dictionary<string, object> ExtensionData { get; set; }
}

I wouldn't necessarily consider this to be a bug, although I can see why the expected behavior would be nice to have. We might want to consider it in a future release. cc @layomia @steveharter

[svick]

This shouldn't have been closed, right?

[Mizuwokiru]

If someone still needs to use this approach, the issue can be avoided by adding JsonPropertyName attribute that doesn't match to any property of your object.
For example:

public class TestDefault
{
    [JsonPropertyName($"${nameof(Value)}"), JsonIgnore]
    public JsonElement Value => Values["value"];

    [JsonExtensionData]
    public Dictionary<string, JsonElement> Values { get; set; }
}

[krwq]

Better workaround would be:

using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;

JsonSerializerOptions options = new()
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
    {
        Modifiers =
        {
            ti =>
            {
                if (ti.Kind == JsonTypeInfoKind.Object)
                {
                    JsonPropertyInfo[] props = ti.Properties
                        .Where(prop => prop.AttributeProvider == null || prop.AttributeProvider.GetCustomAttributes(typeof(JsonIgnoreAttribute), false).Length == 0)
                        .ToArray();

                    if (props.Length != ti.Properties.Count)
                    {
                        ti.Properties.Clear();
                        foreach (var prop in props)
                        {
                            ti.Properties.Add(prop);
                        }
                    }
                }
            }
        }
    }
};

var json = @"{ ""Value"": { ""content"": ""test"" } }";
TestDefault obj = JsonSerializer.Deserialize<TestDefault>(json, options);
if (obj.Values != null)
{
    foreach (var (k, v) in obj.Values)
    {
        Console.WriteLine($"{k} => {v}"); // Value => { "content": "test" }
    }
}

public class TestDefault
{
    [JsonIgnore]
    public JsonElement Value => Values["value"];

    [JsonExtensionData]
    public Dictionary<string, JsonElement>? Values { get; set; }
}

Note 1: ignored properties will also not work with parametrized constructors in that case (assuming that was desired in the first place).

Note 2: It needs testing with inheritance if needed for production