Allow external libraries to extend Results

[sebastienros]

When using minimal APIs, external libraries could want to expose custom results, like views. There is no way to add custom extension methods to the Results class. However a solution could be to expose a public property e.g. Results.Custom that could be used to expose other types of results.

Today the only solution is for each library to create their own factory class, or return an IResult instance directly which doesn't help for discoverability, and consistency.

Example:

app.MapGet("/", () =>
{
    return LiquidResults.View("index", new Todo(1, "Go back to work!", false));
});

(https://github.com/sebastienros/fluid/pull/292/files#diff-b2a1bda3022b9e2130cc1ececbfc576ec1f91dca2c65baef80c1604c8c36821eR11-R14)

Which could instead become:

app.MapGet("/", () =>
{
    return Results.Custom.LiquidView("index", new Todo(1, "Go back to work!", false));
});

[davidfowl]

cc @khellang had similar questions about this.

[DamianEdwards]

We talked about this when we introduced the Results type and the extensibility limitations of the approach, but I feel like this is a worthwhile addition to enable extensions. It's such a trivial addition it really just comes down to whether we're OK committing to this for the long term.

[sebastienros]

Another alternative is to use context or context.Response as the extension method source as long as it's passed to the lambda. Still less discoverable and minimalistic.

[DamianEdwards]

Yeah having to accept HttpContext or HttpResponse is unfortunate just to get access to extended IResult helpers, and doesn't align with what we have for the inbox stuff either. I prefer the idea of Results.Custom being the extension point.

[pranavkm]

Background and Motivation

We would like for library authors to provide custom IResult instances and for a single place to register these extensions for easier discoverability.

Proposed API

namespace Microsoft.AspNetCore.Http.Results
{
+    // Marker interface for extensibility
+    public interface IResultExtensions
+    {
+    }

    public static class Results
    {
+    public static IResultExtensions Extensions { get; } 
    }
}

Usage Examples

// Library code
public static class LiquidResultExtensions
{
   public static IResult LiquidView(this IResultExtensions resultExtensions, string name)
   {
         return new LiquidView(name);
    }
}

// App code
MapGet("/view", () => Results.Extensions.LiquidView("cool-view"));

Alternative Designs

Users can discover result types from libraries through documentation

Risks

N/A

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.

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.

[pranavkm]

API Review

We considered a few options

• Not shipping this and hoping a language feature that supersedes this.
• Using a static holder type e.g.

using static ResultsHolder = Microsoft.AspNetCore.Http;

public static class HttpResults
{
    public static IResults Results { get; }
}

This felt iffy since it's only usable within C# that has global static usings.

At this point, we feel there is a need for an API for the community to rally around, so even if it does get deprecated by a newer language feature, we think it's worthwhile.

Naming:

• Results.Custom
• Results.Contributions

Since the type name has IResultExtensions already has Extensions in it, it felt like keeping that name is ok.

There was some concern about using Extensions as a property name, since there's a chance that it can conflict with a namespace, or if multiple libraries add the same named extension, but we can provide guidance here.