Support using any dictionary as JsonExtensionData

[kostrse]

IMHO, IReadOnlyDictionary should work identical to IDictionary when using with JsonExtensionData attribute.

[JsonExtensionData]
public IReadOnlyDictionary<string, JsonElement> Payload { get; set; }

Currently it fails with error:

System.InvalidOperationException: The data extension property 'Payload' does not match the required signature of IDictionary<string, JsonElement> or IDictionary<string, object>.

This is analogous to collection properties which can be defined as List<T>, IList<T> or IReadOnlyList<T> etc.

[layomia]

Triage: we should generalize this to support all dictionary types that the serializer supports as extension data. Supported types can be found here dotnet/docs#15807 (comment).

Changing the title to reflect this.

[layomia]

Adding request for ImmutableDictionary from @mwikstrom in #32408:

Hi,

I want to use an immutable dictionary to store json extension data.

I tried to see if this is supported by writing the following test in an mstest project targeting netcoreapp3.1:

[TestClass]
public sealed class JsonExtensionDataTest
{
    class HasImmutableExtensionData
    {
        [JsonExtensionData]
        public ImmutableDictionary<string, JsonElement> ExtensionData { get; set; }
    }

    [TestMethod]
    public void Can_deserialize_immutable_extension_data()
    {
        const string json = "{\"foo\":123}";
        var obj = JsonSerializer.Deserialize<HasImmutableExtensionData>(json);
        Assert.IsNotNull(obj.ExtensionData);
        Assert.IsTrue(obj.ExtensionData.ContainsKey("foo"));
    }
}

That test fails with a NullReferenceException which surprised me:

 Can_deserialize_immutable_extension_data
   Source: JsonExtensionDataTest.cs line 42
   Duration: 39 ms

  Message: 
    Test method JsonExtensionDataTest.JsonExtensionDataTest.Can_deserialize_immutable_extension_data threw exception: 
    System.NullReferenceException: Object reference not set to an instance of an object.
  Stack Trace: 
    JsonSerializer.CreateDataExtensionProperty(JsonPropertyInfo jsonPropertyInfo, ReadStack& state)
    JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
    JsonSerializer.ReadCore(Type returnType, JsonSerializerOptions options, Utf8JsonReader& reader)
    JsonSerializer.Deserialize(String json, Type returnType, JsonSerializerOptions options)
    JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
    JsonExtensionDataTest.Can_deserialize_immutable_extension_data() line 45

I consider the experienced behavior a bug. Another exception should have been thrown.

I would also like to request future support for using ImmutableDictionary<TKey, TValue> as backer for [JsonExtensionData].

[Romanx]

Hey there,

I'd love to pick this up and take a look at getting these types serializing. Could the issue be assigned to me?

[layomia]

@Romanx, certainly. Please let me know if you run into any issue or have any questions.