System.Text.Json: JsonExtensionData deserialization silently fails for a get-only, preallocated dictionary property.

[dbc2]

If I have a get-only, preallocated [JsonExtensionData] property on my model, then the dictionary will be serialized successfully, but will silently fail to be populated on deserialization. This seems wrong. Details as follows.

Say I have the following model:

public class Model
{
    [JsonExtensionData]
    public Dictionary<string, object> ExtensionData { get; } = new Dictionary<string, object>();
}

And the following test method:

public class TestClass
{
    [TestMethod]
    public void TestReadOnlyExtensionDataProperty()
    {
        var model1 = new Model { ExtensionData = { { "item", "item value" } } };

        var json = JsonSerializer.Serialize(model1);

        Debug.WriteLine(json); // Prints {"item":"item value"} as expected

        Assert.AreEqual(json, "{\"item\":\"item value\"}"); // Passes.

        var model2 = JsonSerializer.Deserialize<Model>(json);

        Assert.AreEqual(model1.ExtensionData.Count, model2.ExtensionData.Count, "model2.ExtensionData.Count"); // FAILS!
    }
}

Then the first assert will succeed, because model1 is correctly serialized as {"item":"item value"}. However, the second assert fails, because model2.ExtensionData is empty when deserialized.

Demo fiddle 1 here reproducing the problem.

Conversely, Json.NET is able to round-trip this model successfully, see demo fiddle 2 here.

And if I modify Model.ExtensionData to have a setter that throws an exception, then deserialization also succeeds:

public class Model
{
    readonly Dictionary<string, object> extensionData = new Dictionary<string, object>();

    [JsonExtensionData]
    public Dictionary<string, object> ExtensionData { get => extensionData; set => throw new NotImplementedException(); }
}

Demo fiddle 3 here, which passes.

This seems wrong. Since the dictionary is preallocated it should be populated successfully during deserialization even without a setter -- especially since the setter isn't actually called.

[layomia]

This is a duplicate of #30258. For now, like any other property, the extension data property needs a setter to be deserialized into.