Shouldn't a JsonIgnore property be included in JsonExtensionData property during deserialzation?

[SilentCC]

According to the definition of JsonExtensionData:

When placed on a property of type IDictionary<TKey,TValue>, any properties that do not have a matching member are added to that dictionary during deserialization and written during serialization.

When deserializing a property with JsonIgnore attribute, shouldn't it be a mismatching member because we ignore the property?

Currently, System.Text.Json doesn't fill the JsonExtensionData property with a JsonIngore property:
This is the unit test function in ExtensionDataTest

public class ClassWithIgnoredData
{
    [JsonExtensionData]
    public Dictionary<string, object> MyOverflow { get; set; }

    [JsonIgnore]
    public int MyInt { get; set; }
}

[Fact]
public async Task IgnoredDataShouldNotBeExtensionData()
{
    ClassWithIgnoredData obj = await Serializer.DeserializeWrapper<ClassWithIgnoredData>(@"{""MyInt"":1}");

    Assert.Equal(0, obj.MyInt);
    Assert.Null(obj.MyOverflow);
}

[ghost]

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

---

According to the definition of JsonExtensionData:

When placed on a property of type IDictionary<TKey,TValue>, any properties that do not have a matching member are added to that dictionary during deserialization and written during serialization.

When deserializing a property with JsonIgnore attribute, shouldn't it be a mismatching member because we ignore the property?

Currently, System.Text.Json doesn't fill the JsonExtensionData property with a JsonIngore property:
This is the unit test function in ExtensionDataTest

 public class ClassWithIgnoredData
        {
            [JsonExtensionData]
            public Dictionary<string, object> MyOverflow { get; set; }

            [JsonIgnore]
            public int MyInt { get; set; }
        }

        [Fact]
        public async Task IgnoredDataShouldNotBeExtensionData()
        {
            ClassWithIgnoredData obj = await Serializer.DeserializeWrapper<ClassWithIgnoredData>(@"{""MyInt"":1}");

            Assert.Equal(0, obj.MyInt);
            Assert.Null(obj.MyOverflow);
        }

Author:	SilentCC
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[krwq]

I agree this seems counter intuitive (at least to me). I'd expect whatever is not being used to go to extension data but at the same time:

• changing that will be a breaking change and will require documenting that in multiple places (although IMO there will be very little people if any at all relying on that)
• most likely if we change JsonIgnore-d properties to be completely ignored by parser we should do the same for parametrized ctors - ignored properties should not be matched with parameters on ctor if we go this path

[gregsdennis]

To voice a contrary opinion, I would expect MyInt not to appear in the extension data as it's been explicitly ignored.

The reasoning for this is that we've indicated in the model that we know the property should be in the data and that it should be ignored if it is.

[krwq]

Closing as duplicate of #68895

@gregsdennis fair but currently contract model doesn't expose IsIgnored and IsIgnored was supposed to mean the same as not present in the model. I can see translating ignored as properties without getter or setter which would be equivalent to current behavior. The problem starts appearing when we have properties with same name on the derived class but not ignoring the property. Let's continue discussion in the other issue if needed