[API Proposal]: Add generic versions of System.Text.Json.Serialization attributes

[jhartmann123]

Background and motivation

With the introduction of generic attributes, some attributes in System.Text.Json.Serialization are good candidates for generic versions, as all public constructors require a type parameter, namely:

• JsonConverterAttribute
• JsonDerivedTypeAttribute (introduced in Developers can use System.Text.Json to serialize type hierarchies securely #63747)

API Proposal

namespace System.Text.Json

public class JsonConverterAttribute<T> : JsonConverterAttribute
{
    public JsonConverterAttribute() : base(typeof(T))
    { }
}

public class JsonDerivedTypeAttribute<T> : JsonDerivedTypeAttribute
{
    public JsonDerivedTypeAttribute() : base(typeof(T))
    { }

    public JsonDerivedTypeAttribute(string typeDiscriminator) : base(typeof(T), typeDiscriminator)
    { }

    public JsonDerivedTypeAttribute(int typeDiscriminator) : base(typeof(T), typeDiscriminator)
    { }
}

API Usage

[JsonDerivedType<Derived>()]
public class Base
{
    public int X { get; set; }
}

public class Derived : Base
{
    public int Y { get; set; }
}

Alternative Designs

No response

Risks

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

---

Background and motivation

With the introduction of generic attributes, some attributes System.Text.Json.Serialization are good candidates for generic generic versions, as all public constructors require a type parameter, namely:

• JsonConverterAttribute
• JsonDerivedTypeAttribute (introduced in Developers can use System.Text.Json to serialize type hierarchies securely #63747)

API Proposal

namespace System.Text.Json

public class JsonConverterAttribute<T> : JsonConverterAttribute
{
    public JsonConverterAttribute() : base(typeof(T))
    { }
}

public class JsonDerivedTypeAttribute<T> : JsonDerivedTypeAttribute
{
    public JsonDerivedTypeAttribute() : base(typeof(T))
    { }

    public JsonDerivedTypeAttribute(string typeDiscriminator) : base(typeof(T), typeDiscriminator)
    { }

    public JsonDerivedTypeAttribute(int typeDiscriminator) : base(typeof(T), typeDiscriminator)
    { }
}

API Usage

[JsonDerivedType<Derived>()]
public class Base
{
    public int X { get; set; }
}

public class Derived : Base
{
    public int Y { get; set; }
}

Alternative Designs

No response

Risks

No response

Author:	jhartmann123
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json
Milestone:	-

[PaulusParssinen]

I tried this for JsonSerializableAttribute here: #71712

[eiriktsarpalis]

Closing as duplicate of #71712. See #71712 (comment) for a justification.

[eiriktsarpalis]

Reopening for consideration in the future.

[IEvangelist]

Thanks for reconsidering this @eiriktsarpalis!

[eiriktsarpalis]

Possible API shape:

namespace System.Text.Json

public class JsonConverterAttribute<TConverter> : JsonConverterAttribute
    where TConverter : JsonConverter
{
    public JsonConverterAttribute();
}

public class JsonDerivedTypeAttribute<TDerivedType> : JsonDerivedTypeAttribute
     where TDerivedType : class
{
    public JsonDerivedTypeAttribute();
    public JsonDerivedTypeAttribute(string typeDiscriminator);
    public JsonDerivedTypeAttribute(int typeDiscriminator);
}

public class JsonSerializableAttribute<T> : JsonSerialiableAttribute
{
    public class JsonSerializableAttribute();
}

[aradalvand]

It's strange this didn't make it to .NET 8 at least, given how trivial it would be to add.

It's the sort of thing you'd intuitively expect to be there at this point, since C# has supported generic attributes for a while now; it's always ugly to have [SomeAttribute(typeof(SomeType)] usages in general.

[eiriktsarpalis]

It's the sort of thing you'd intuitively expect to be there at this point, since C# has supported generic attributes for a while now; it's always ugly to have [SomeAttribute(typeof(SomeType)] usages in general.

From a cost-benefit perspective adding generic variants of existing attributes introduces duplication while not offering a whole lot of benefit. Other than saving the need to type five characters, they mostly don't enhance type safety and they cannot be used in .NET languages that don't support generic attributes. We've kept this open for future consideration, although it doesn't register very high in our priorities for the time being.

[aradalvand]

introduces duplication while not offering a whole lot of benefit.

This reasoning makes no sense; the "duplication" (if we can call it that) that it introduces is minimal and the benefit is just better syntax and ehanced type-safety.

they mostly don't enhance type safety

They do though, you can pass any type to the constructor of JsonConverterAttribute (e.g. [JsonConverter(typeof(int))]) while the generic version would enforce that it's a JsonConverter.

[eiriktsarpalis]

they mostly don't enhance type safety

They do though, you can pass any type to the constructor of JsonConverterAttribute (e.g. [JsonConverter(typeof(int))]) while the generic version would enforce that it's a JsonConverter.

It wouldn't do anything when it comes to enforcing the right converter type though, so I might be annotating a property of type int with a JsonConverter<string> and the generic attribute would be none the wiser. It's true that it would be rejecting [JsonConverter<int>] annotations, but that's just guarding against pathological inputs -- I don't see how it could enhance type safety in real terms.

FWIW it's fairly straightforward to define generic attributes in your code, if you need them:

public class MyPoco
{
    [JsonConverter<JsonStringEnumConverter>]
    public BindingFlags Value { get; set; }
}

public class JsonConverterAttribute<T>() : JsonConverterAttribute(typeof(T))
    where T : JsonConverter
{ }