Add a result filter for razor pages

[haacked]

Right now, there's no nice declarative way (that I could find) to add a feature flag to a razor page.

[FeatureGate(FeatureFlags.FeatureA)]
public class IndexModel : PageModel {...}

Doesn't work because FeatureGateAttribute only overrides OnActionExecutionAsync. I haven't tested this myself, but I believe if you also implement OnResultExecutingAsync you'll cover Razor pages. At least according to the docs Filter methods for Razor Pages in ASP.NET Core

[haacked]

Just as a follow-up, I hacked up a proof of concept to show this would work.

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class FeatureToggleAttribute : FeatureGateAttribute
{
    public FeatureToggleAttribute(params string[] features)
        : this(RequirementType.All, features)
    {
    }

    public FeatureToggleAttribute(RequirementType requirementType, params string[] features)
        : base(requirementType, features)
    {
    }

    public FeatureToggleAttribute(RequirementType requirementType, params object[] features)
        : base(requirementType, features)
    {
    }
    
    public FeatureToggleAttribute(params object[] features)
        : this(RequirementType.All, features)
    {
    }

    public override async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
    {
        var fm = context.HttpContext.RequestServices.GetRequiredService<IFeatureManagerSnapshot>();
        bool flag;
        if (RequirementType == RequirementType.All)
            flag = await Features.All<string>((Func<string, Task<bool>>) (async feature =>
                await fm.IsEnabledAsync(feature).ConfigureAwait(false)));
        else
            flag = await Features.Any<string>((Func<string, Task<bool>>) (async feature =>
                await fm.IsEnabledAsync(feature).ConfigureAwait(false)));
        if (flag)
        {
            await next().ConfigureAwait(false);
        }
        else
        {
            context.HttpContext.Response.StatusCode = 404;
            await context.HttpContext.Response.CompleteAsync();
        }
    }
}

This code makes use of internal classes within this repo that I didn't copy. But it shows that the basic idea would work.

[ArieJones]

+1 for this as this is exactly what we were looking at as a solution in one of our apps ..

[jimmyca15]

I spent some time looking into this because I wanted to provide some out of the box support for Razor Pages in the same way that we provide it for MVC controllers. There is an unfortunate difference between supporting controllers via OnActionExecutionAsync and support Razor Pages via OnResultExecutionAsync. OnResultExecutionAsync is invoked after a page handler has already executed. You are correct that this is the only way to declaratively add a handler to a Page Handler. If we wanted to support Razor Pages with just an attribute, we would have to take that route, but the intent of the FeatureGate attribute is to prevent the action/handler from running if the feature is not enabled. Therefore, it looks like support for this may not be a good option with what is available directly out of ASP.NET Core.

I will look a bit more to see if it makes sense to provide a global IPageFilter that can check if a page is decorated with a FeatureGateAttribute and execute it before the handler similar to how it works in an MVC scenario. This may require expensive reflection though and additional boiler plate code on startup to add the global filter.

[haacked]

Good point on the result execution filter being too late.

Could FeatureGateAttribute itself implement IPageFilter and IAsyncPageFilter?

[mdarefull]

@jimmyca15 IMO, this would really increase the value of this library and shouldn't be difficult to implement.

We already implemented a a very simple IAsyncPageFilter with similar implementation provided by @haacked and it's nicely working:

    [AttributeUsage(AttributeTargets.Class)]
    public class RazorPageFeatureGate : Attribute, IAsyncPageFilter
    {
        private string Feature { get; }
        public RazorPageFeatureGate(string feature)
            => Feature = feature;

        public virtual async Task OnPageHandlerExecutionAsync(PageHandlerExecutingContext context, PageHandlerExecutionDelegate next)
        {
            var fm = context.HttpContext.RequestServices.GetRequiredService<IFeatureManagerSnapshot>();
            var isEnabled = await fm.IsEnabledAsync(Feature).ConfigureAwait(false);
            if (isEnabled)
            {
                await next.Invoke().ConfigureAwait(false);
            }
            else
            {
                context.HttpContext.Response.StatusCode = 404;
                await context.HttpContext.Response.CompleteAsync().ConfigureAwait(false);
            }
        }

        public virtual Task OnPageHandlerSelectionAsync(PageHandlerSelectedContext context) => Task.CompletedTask;
    }

[imsaha]

 services
               .AddRazorPages(options =>
               {
                   ...
                  options.Conventions.AddAreaFolderApplicationModelConvention("Admin", "/Customers", cOption =>
                   {
                       cOption.Filters.Add(new RazorPageFeatureGate(ApplicationFeatureFlags.ManageCustomer));
                   });
               })

Apply feature gate filter to area folder

[MarcWils]

Calling Complete on the response also disables custom error pages.

context.HttpContext.Response.StatusCode = 404;
await context.HttpContext.Response.CompleteAsync().ConfigureAwait(false);

Wouldn't it be better to avoid page execution by setting the PageHandlerExecutingContext.Result?

context.Result = new NotFoundResult();

[CarlVerret]

We already implemented a a very simple IAsyncPageFilter with similar implementation provided by @haacked and it's nicely working:

I suggest you make a PR from this. Worked like a charm ! Thanks for sharing

[jimmyca15]

Since page filters and action filters are executed in isolation we can add this new functionality in the existing FeatureGateAttribute. I have a sent out a PR #166.

[jimmyca15]

This has been released in Microsoft.FeatureManagement.AspNetCore 2.5.0

Documentation update is in the works.