System.Text.Json unable to deserialize an object that implement IEnumerable but is itself not a list

[RyanGaudion]

Description

JsonSerializer.Deserialize is unable to deserialize a class that contains it's own properties will also implementing IEnumerable

Reproduction Steps

Please see the test code here:

    public class UnitTest1
    {
        [Fact]
        public void Test1()
        {
            string validJson = @"
{
    ""List1"": [{""Test"": ""a""}],
    ""List2"": [{""Test"": ""a""}]
}";

            MyClass obj = JsonSerializer.Deserialize<MyClass>(validJson);

        }
    }
    class MyClass //: IEnumerable<TestClass>
    {
        public List<TestClass> List1 { get; set; } = new List<TestClass>();
        public List<TestClass> List2 { get; set; } = new List<TestClass>();

        public IEnumerator<TestClass> GetEnumerator()
        {
            return List1.Concat(List2).GetEnumerator();
        }
        //IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }

    class TestClass
    {
        public string Test { get; set; }
    }

This code passes, but when uncommenting out the 2 comments in MyClass - the tests then fail.

Expected behavior

I expected adding an interface and method to an object to not effect it's ability to be deserialized

Actual behavior

The deserialization fails with this error System.Text.Json.JsonException : The JSON value could not be converted to MyClass. Path: $ | LineNumber: 1 | BytePositionInLine: 1.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

Description

JsonSerializer.Deserialize is unable to deserialize a class that contains it's own properties will also implementing IEnumerable

Reproduction Steps

Please see the test code here:

    public class UnitTest1
    {
        [Fact]
        public void Test1()
        {
            string validJson = @"
{
    ""List1"": [{""Test"": ""a""}],
    ""List2"": [{""Test"": ""a""}]
}";

            MyClass obj = JsonSerializer.Deserialize<MyClass>(validJson);

        }
    }
    class MyClass //: IEnumerable<TestClass>
    {
        public List<TestClass> List1 { get; set; } = new List<TestClass>();
        public List<TestClass> List2 { get; set; } = new List<TestClass>();

        public IEnumerator<TestClass> GetEnumerator()
        {
            return List1.Concat(List2).GetEnumerator();
        }
        //IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }

    class TestClass
    {
        public string Test { get; set; }
    }

This code passes, but when uncommenting out the 2 comments in MyClass - the tests then fail.

Expected behavior

I expected adding an interface and method to an object to not effect it's ability to be deserialized

Actual behavior

The deserialization fails with this error System.Text.Json.JsonException : The JSON value could not be converted to MyClass. Path: $ | LineNumber: 1 | BytePositionInLine: 1.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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

[krwq]

You'll need to implement CustomConverter for your type.

The reason this is not serializable is following:

• anything implementing IEnumerable will be by default serialized/deserialized as an array and not as object - currently there is no good workaround for that (Developers should have access to System.Text.Json's default internal converters #63791 or System.Text.Json converter infrastructure improvements for .NET 8 #77018 should fix that but it's not planned for near future)
• deserializer doesn't know how to add values to your collection (since you're implementing IEnumerable assumption is that it should deserialize from array and deserializer needs to add elements somehow)

This issue has been marked needs-author-action and may be missing some important information.

[krwq]

Marking as needs-author-action because given current information we should close this issue as duplicate. Please close or add further details (bot will automatically close this issue if there is no response)

[RyanGaudion]

@krwq thank you for the reference to #63791. This looks like it'll resolve the issue. Is there any workaround to get an implementation like #63791 working with a custom converter? If so, what would this converter look like?

[krwq]

Implementation could look something like this (done very minimal testing on that, just showing idea):

class MyClassConverter : JsonConverter<MyClass>
{
    public override MyClass? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        MyClass obj = new();

        while (reader.TokenType != JsonTokenType.EndObject)
        {
            if (reader.TokenType == JsonTokenType.PropertyName)
            {
                string? propertyName = reader.GetString();
                reader.Read();
                List<TestClass>? list = JsonSerializer.Deserialize<List<TestClass>>(ref reader, options);
                if (propertyName == nameof(MyClass.List1))
                {
                    obj.List1 = list;
                }
                else if (propertyName == nameof(MyClass.List2))
                {
                    obj.List2 = list;
                }
                else
                {
                    // we ignore unknown properties by default
                }
            }

            reader.Read();
        }

        return obj;
    }

    public override void Write(Utf8JsonWriter writer, MyClass value, JsonSerializerOptions options)
    {
        writer.WriteStartObject();
        writer.WritePropertyName(nameof(MyClass.List1));
        JsonSerializer.Serialize(writer, value.List1, options);
        writer.WritePropertyName(nameof(MyClass.List2));
        JsonSerializer.Serialize(writer, value.List2, options);
        writer.WriteEndObject();
    }
}

then you add that to options which you pass to Serialize/Deserialize:

JsonSerializerOptions options = new()
{
    Converters = { new MyClassConverter() }
};