Attribute foo used in [JsonPolymorphic(TypeDiscriminatorPropertyName = "foo")] does not get deserialized

[sebader]

Description

I'm deserializing the followed JSON structure:

"data": {
        "resultType": "vector",  // or: "matrix"
        "result": [
            {
                // ...
            }
        ]
    }

For this I have build my polymorphic objects as follows:

    [JsonPolymorphic(TypeDiscriminatorPropertyName = "resultType")]
    [JsonDerivedType(typeof(Data))]
    [JsonDerivedType(typeof(MatrixData), typeDiscriminator: "matrix")]
    [JsonDerivedType(typeof(VectorData), typeDiscriminator: "vector")]
    public class Data
    {
        [JsonPropertyName("resultType")]
        public string resultType { get; set; }
    }

    public class VectorData : Data
    {
// ...
    }

    public class MatrixData : Data
    {
// ...
    }

The deserialization including polymorphism all works great, what does not work is the property resultType. It is always null.

So, I'm wondering if its use in [JsonPolymorphic(TypeDiscriminatorPropertyName = "resultType")] is messing with the deserialization of that field?!

Reproduction Steps

See above

Expected behavior

Any field that is being used for JsonPolymorphic should still be able to be deserialized

Actual behavior

Field is null

Regression?

No response

Known Workarounds

No response

Configuration

.NET SDK:
Version: 7.0.101
Commit: bb24aafa11

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22623
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\7.0.101\

Other information

No response

[ghost]

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Issue Details

---

Description

I'm deserializing the followed JSON structure:

"data": {
        "resultType": "vector",  // or: "matrix"
        "result": [
            {
                // ...
            }
        ]
    }

For this I have build my polymorphic objects as follows:

    [JsonPolymorphic(TypeDiscriminatorPropertyName = "resultType")]
    [JsonDerivedType(typeof(Data))]
    [JsonDerivedType(typeof(MatrixData), typeDiscriminator: "matrix")]
    [JsonDerivedType(typeof(VectorData), typeDiscriminator: "vector")]
    public class Data
    {
        [JsonPropertyName("resultType")]
        public string resultType { get; set; }
    }

    public class VectorData : Data
    {
// ...
    }

    public class MatrixData : Data
    {
// ...
    }

The deserialization including polymorphism all works great, what does not work is the property resultType. It is always null.

So, I'm wondering if its use in [JsonPolymorphic(TypeDiscriminatorPropertyName = "resultType")] is messing with the deserialization of that field?!

Reproduction Steps

See above

Expected behavior

Any field that is being used for JsonPolymorphic should still be able to be deserialized

Actual behavior

Field is null

Regression?

No response

Known Workarounds

No response

Configuration

.NET SDK:
Version: 7.0.101
Commit: bb24aafa11

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22623
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\7.0.101\

Other information

No response

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

[pinkfloydx33]

This came up recently but I cannot find the issue. We had a similar problem. The fact it has a setter implies you can change the underlying type by changing the value, which wouldn't make sense. We were using the property for introspection without a type-check and originally resolved it by putting JsonIgnore on the property (to prevent double serialization) and turned it into an abstract, get-only auto-property that returned the expected value by type, which is a variation on what was recommended by dotnet folks.

We ended up just dropping the property (which was entirely a convenience anyways) and just perform the type check ourself now.

[sebader]

yeah its not a super hard requirement for me to have this, either, but it just felt like a bug that it's always null. If that is indeed the desired behavior, it should at least be documented on JsonPolymorphic

[eiriktsarpalis]

This is by design, the type discriminator identifies types and so cannot be mapped to a run time property value.