Polymorphic deserialization requires disriminator property order to be first in json.

[vladimir-ilnytskyi]

Description

If discriminator property is not placed as a FIRST property then polymorphic object converter doesn't find it.
As json property order might be any, it puts a big inconvenience for example for frontend to actually send discriminator as first property
{ "nonDiscrimProperty": 1, "discrimProperty": 1 } // discrimProperty is discriminator

Second inconvenience is that i have to use JsonIgnore attribute on the object if i want this property to be present in C# class.

Reproduction Steps

`
[JsonDerivedType(typeof(Cat), 1)]
[JsonPolymorphic(TypeDiscriminatorPropertyName = nameof(Type))]
public abstract class Animal
{
public string Name { get; set; }
[JsonIgnore] // workaround, serialize duplicates discriminator in json and makes json invalid
public virtual int Type { get; set; }
}

public class Cat : Animal
{
public override int Type => 1;
}

var animal = new Cat { Name = "cat" };
var array = new Animal[] { animal };
var json = JsonSerializer.Serialize(array); // polymorphically serialize array of animals
var obj = JsonSerializer.Deserialize<Animal[]>(json); // ok

var obj2 = JsonSerializer.Deserialize<Animal[]>("[{"Type": 1}]"); // ok
var obj3 = JsonSerializer.Deserialize<Animal[]>("[{"Name":"mycat","Type": 1}]"); // fails because converter count find discriminator as first property`

Expected behavior

`var obj3 = JsonSerializer.Deserialize<Animal[]>("[{"Name":"mycat","Type": 1}]"); // should deserialize the object``

Actual behavior

`var obj3 = JsonSerializer.Deserialize<Animal[]>("[{"Name":"mycat","Type": 1}]"); // fails because converter count find discriminator as first property``

Regression?

No response

Known Workarounds

force all parties to send discriminator as first object.
use jsonignore attribute if you dont want your property to be duplicated with the discriminator during serialization.
or [JsonDerivedType(typeof(Animal))] to exclude discriminator from serialization

Configuration

system.text.json 7.0.0

Other information

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

If discriminator property is not placed as a FIRST property then polymorphic object converter doesn't find it.
As json property order might be any, it puts a big inconvenience for example for frontend to actually send discriminator as first property
{ "nonDiscrimProperty": 1, "discrimProperty": 1 } // discrimProperty is discriminator

Second inconvenience is that i have to use JsonIgnore attribute on the object if i want this property to be present in C# class.

Reproduction Steps

`
[JsonDerivedType(typeof(Cat), 1)]
[JsonPolymorphic(TypeDiscriminatorPropertyName = nameof(Type))]
public abstract class Animal
{
public string Name { get; set; }
[JsonIgnore] // workaround, serialize duplicates discriminator in json and makes json invalid
public virtual int Type { get; set; }
}

public class Cat : Animal
{
public override int Type => 1;
}

var animal = new Cat { Name = "cat" };
var array = new Animal[] { animal };
var json = JsonSerializer.Serialize(array); // polymorphically serialize array of animals
var obj = JsonSerializer.Deserialize<Animal[]>(json); // ok

var obj2 = JsonSerializer.Deserialize<Animal[]>("[{"Type": 1}]"); // ok
var obj3 = JsonSerializer.Deserialize<Animal[]>("[{"Name":"mycat","Type": 1}]"); // fails because converter count find discriminator as first property`

Expected behavior

`var obj3 = JsonSerializer.Deserialize<Animal[]>("[{"Name":"mycat","Type": 1}]"); // should deserialize the object``

Actual behavior

`var obj3 = JsonSerializer.Deserialize<Animal[]>("[{"Name":"mycat","Type": 1}]"); // fails because converter count find discriminator as first property``

Regression?

No response

Known Workarounds

force all parties to send discriminator as first object.
use jsonignore attribute if you dont want your property to be duplicated with the discriminator during serialization.

Configuration

system.text.json 7.0.0

Other information

No response

Author:	vladimir-ilnytskyi
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[Tornhoof]

See #72604

[vladimir-ilnytskyi]

@Tornhoof thanks! seems like a dup, but according to json spec it is NORMAL to have random order of props...
I don't get how do why do i have to force browser to send ordered properties =(

[layomia]

Duplicate of #72604.