Add generic versions of attributes in ASP.NET Core

[DamianEdwards]

Background and Motivation

Now that C# supports generic attributes as of version 10/.NET 6, we should look through ASP.NET Core and consider adding generic versions of any attributes that accept type parameters (e.g. ConsumesAttribute).

Proposed API

namespace Microsoft.AspNetCore.Mvc;

//Original: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/ModelMetadataTypeAttribute.cs
+[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
+public class ModelMetadataTypeAttribute<TType> : ModelMetadataTypeAttribute
+{
+    public ModelMetadataTypeAttribute() : base(typeof(TType))
+    { }
+}

// Original: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/Filters/MiddlewareFilterAttribute.cs
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+public class MiddlewareFilterAttribute<TConfigurationType> : MiddlewareFilterAttribute
+{
+    public MiddlewareFilterAttribute() : base(typeof(TConfigurationType))
+    { }
+}

// Original: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/ServiceFilterAttribute.cs
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+[System.Diagnostics.DebuggerDisplay("ServiceFilter: Type={ServiceType} Order={Order}")]
+public class ServiceFilterAttribute<TFilterType> : ServiceFilterAttribute
+ where TFilterType : IFilterMetadata
+{
+    public ServiceFilterAttribute() : base(typeof(TFilterType))
+    { }
+}

// Original: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/ModelBinderAttribute.cs
+[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Struct, AllowMultiple = false, Inherited = true)]
+public class ModelBinderAttribute<TBinderType> : ModelBinderAttribute
+ where TBinderType : IModelBinder
+{
+    public ModelBinderAttribute() : base(typeof(TBinderType))
+    { }
+}

// Original: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/ProducesAttribute.cs
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+public class ProducesAttribute<TType> : ProducesAttribute
+{
+    public ProducesAttribute() : base(typeof(TType))
+    { }
+}

// Original: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/ProducesResponseTypeAttribute.cs
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+public class ProducesResponseTypeAttribute<TType> : ProducesResponseTypeAttribute
+{
+    public ProducesResponseTypeAttribute(int statusCode) : base(typeof(TType), statusCode)
+    { }
+    public ProducesResponseTypeAttribute(int statusCode, string contentType, params string[] additionalContentTypes)
+             : base(typeof(TType), statusCode, contentType, additionalContentTypes)
+    { }
+}

In addition to what listed above the original list (#37767 (comment)) contains:

src/Mvc/Mvc.Razor/src/Compilation/RazorViewAttribute.cs:
  13:     public class RazorViewAttribute : Attribute

src/Razor/Microsoft.AspNetCore.Razor.Language/src/ProvideRazorExtensionInitializerAttribute.cs:
  9:     public class ProvideRazorExtensionInitializerAttribute : Attribute

RazorViewAttribute is marked as obsolete, so, I don't think we need to do anything (cc @mkArtakMSFT in case you want to have it added).
Also, ProvideRazorExtensionInitializerAttribute is not part of the ASPNET.Core repo anymore.

Usage Examples

[ProducesResponseType<ValidationProblemDetails>(StatusCodes.Status400BadRequest)]
[ProducesResponseType<Todo>(StatusCodes.Status201Created)]
[EndpointName(nameof(AddTodo))]
[Tags("TodoApi")]
async Task<IResult> AddTodo(Todo todo, TodoDb db)
{
    if (!MiniValidator.TryValidate(todo, out var errors))
        return Results.ValidationProblem(errors);

    db.Todos.Add(todo);
    await db.SaveChangesAsync();

    return Results.Created($"/todos/{todo.Id}", todo);
}

Alternative Designs

Risks

Very small, this will introduce new types derived from the non-generic ones.

[javiercn]

WHAT!! how could I have missed this!!

This is great!

[DamianEdwards]

There may still be some issues with the implementation it turns out: dotnet/csharplang#124 (comment)

[DamianEdwards]

Confirmed, this is still behind the preview language version in .NET 6:

[ShreyasJejurkar]

Did a regex search with public class [a-zA-Z]+ : Attribute throughout the repo for attribute taking Type as the parameter.

9 results - 9 files

src/Mvc/Mvc.Core/src/ModelBinderAttribute.cs:
  28:     public class ModelBinderAttribute : Attribute, IModelNameProvider, IBinderTypeProviderMetadata

src/Mvc/Mvc.Core/src/ModelMetadataTypeAttribute.cs:
  12:     public class ModelMetadataTypeAttribute : Attribute

src/Mvc/Mvc.Core/src/ProducesAttribute.cs:
  21:     public class ProducesAttribute : Attribute, IResultFilter, IOrderedFilter, IApiResponseMetadataProvider

src/Mvc/Mvc.Core/src/ProducesResponseTypeAttribute.cs:
  16:     public class ProducesResponseTypeAttribute : Attribute, IApiResponseMetadataProvider

src/Mvc/Mvc.Core/src/ServiceFilterAttribute.cs:
  25:     public class ServiceFilterAttribute : Attribute, IFilterFactory, IOrderedFilter

src/Mvc/Mvc.Core/src/TypeFilterAttribute.cs:
  27:     public class TypeFilterAttribute : Attribute, IFilterFactory, IOrderedFilter

src/Mvc/Mvc.Core/src/Filters/MiddlewareFilterAttribute.cs:
  15:     public class MiddlewareFilterAttribute : Attribute, IFilterFactory, IOrderedFilter

src/Mvc/Mvc.Razor/src/Compilation/RazorViewAttribute.cs:
  13:     public class RazorViewAttribute : Attribute

src/Razor/Microsoft.AspNetCore.Razor.Language/src/ProvideRazorExtensionInitializerAttribute.cs:
  9:     public class ProvideRazorExtensionInitializerAttribute : Attribute

I have excluded the attributes that are part of the tests project.

[mkArtakMSFT]

@rafikiassumani-msft moving this over to your area path. FYI

[yecril71pl]

This could prevent wrong code from being compiled. Currently you get an exception if the parameter binderType does not implement the interface IModelBinder 👎
Of course, you do not exactly need generic attributes for this, the work-around would be to use a generic type carrier as a parameter rather than a bare Type.

[captainsafia]

Triage: @brunolins16 will take the content from this comment and craft an API proposal template for it.

[ghost]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[BrennanConroy]

API Review Notes:

• We prefer T over TType for generic arguments, unless they are constrained, then we add a suffix to T.

API Approved!

namespace Microsoft.AspNetCore.Mvc;

+[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
+public class ModelMetadataTypeAttribute<T> : ModelMetadataTypeAttribute
+{
+    public ModelMetadataTypeAttribute() : base(typeof(T))
+    { }
+}

+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+public class MiddlewareFilterAttribute<T> : MiddlewareFilterAttribute
+{
+    public MiddlewareFilterAttribute() : base(typeof(T))
+    { }
+}

+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+[System.Diagnostics.DebuggerDisplay("ServiceFilter: Type={ServiceType} Order={Order}")]
+public class ServiceFilterAttribute<TFilter> : ServiceFilterAttribute
+ where TFilter : IFilterMetadata
+{
+    public ServiceFilterAttribute() : base(typeof(TFilter))
+    { }
+}

+[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Struct, AllowMultiple = false, Inherited = true)]
+public class ModelBinderAttribute<TBinder> : ModelBinderAttribute
+ where TBinder : IModelBinder
+{
+    public ModelBinderAttribute() : base(typeof(TBinder))
+    { }
+}

+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+public class ProducesAttribute<T> : ProducesAttribute
+{
+    public ProducesAttribute() : base(typeof(T))
+    { }
+}

+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+public class ProducesResponseTypeAttribute<T> : ProducesResponseTypeAttribute
+{
+    public ProducesResponseTypeAttribute(int statusCode) : base(typeof(T), statusCode)
+    { }

+    public ProducesResponseTypeAttribute(int statusCode, string contentType, params string[] additionalContentTypes)
+             : base(typeof(T), statusCode, contentType, additionalContentTypes)
+    { }
+}

[captainsafia]

While working on implementing this, I found one more type that needed generic attribute overloads.

+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+public class TypeFilterAttribute<TFilter> : TypeFilterAttribute where TFilter : IFilterMetadata
+{
+    public TypeFilterAttribute() : base(typeof(T)) { }
+}

[BrennanConroy]

Looks like it needs to be constrained to IFilterMetadata as well.

[captainsafia]

Looks like it needs to be constrained to IFilterMetadata as well.

Indeed! I updated the comment to reflect this.