Expose Use(Func<RequestDelegate, RequestDelegate> middleware) on WebApplication

[davidfowl]

Background and Motivation

This is the core method for adding middleware to the pipeline that we hid (unintentionally) on the WebApplication because it had an explicit implementation of IApplicationBuilder. Today this is possible but you need to cast to IApplictionBuilder first.

Proposed API

namespace Microsoft.AspNetCore.Builder
{
    public static class WebApplication
    {
+       public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware);
    }
}

Usage Examples

var app = WebApplication.Create();

app.Use(next =>
{
    // Passthrough
    return next;
});

Alternatives

var app = WebApplication.Create();

app.Use((context, next) =>
{
    // Passthrough
    return next(context);
});

The difference here is that there's no place to write the code that runs at startup. Middleware is broken into 2 phases:

1. Construction of the middleware
2. Execution of the request

app.Use(next =>
{
    // This runs on middleware construction
    return context => 
    {
           // This runs per request
          return next(context);
    };
});

Risks

None

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 think it's worthwhile having this, although I personally don't think it adds a great deal of value. That said, we already have two additional overloads, so a 3rd one isn't over kill. Approved.