如何在 app.UseEndPoints 中将自定义路由操作作为委托实例添加到 ASP.NET Core 3.1 应用程序？

Question

I'm trying to use an local delegate instance when mapping route against its handler, namely a function that returns IActionResult , like a regular Action defined in a Controller .

Here is how I have tried:

using System.Globalization;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Localization;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.AspNetCore.Mvc.Infrastructure;
using Microsoft.AspNetCore.Routing;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorPages();

        services.AddLocalization(options => options.ResourcesPath = "Resources");

        services.AddMvcCore();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Error");
        }

        var supportedCultures = new[]
        {
            new CultureInfo("en-US"),
            new CultureInfo("zh-CN"),
            new CultureInfo("ja-JP"),
        };
        app.UseRequestLocalization(new RequestLocalizationOptions
        {
            DefaultRequestCulture = new RequestCulture("en-US"),
            SupportedCultures = supportedCultures,
            SupportedUICultures = supportedCultures
        });

        app.UseStaticFiles();

        app.UseRouting();

        app.UseAuthorization();

        RouteHandler actionA = () => new ContentResult {Content = "Hello A!"};
        ContentRouteHandler actionB = () => new ContentResult {Content = "Hello B!"};
        app.UseEndpoints(endpoints =>
        {
            endpoints.Map("HelloA", context => WriteActionResult(context, actionA()));
            endpoints.Map("HelloB", context => WriteActionResult(context, actionB()));
        });
    }

    public static Task WriteActionResult<TResult>(HttpContext context, TResult result) where TResult : IActionResult
    {
        var executor = context.RequestServices.GetRequiredService<IActionResultExecutor<TResult>>();
        var routeData = context.GetRouteData() ?? new RouteData();
        var actionContext = new ActionContext(context, routeData, new ActionDescriptor());

        return executor.ExecuteAsync(actionContext, result);
    }

    public delegate IActionResult RouteHandler();
    public delegate ContentResult ContentRouteHandler();
}

The core part is

RouteHandler actionA = () => new ContentResult {Content = "Hello A!"};
ContentRouteHandler actionB = () => new ContentResult {Content = "Hello B!"};
app.UseEndpoints(endpoints =>
{
    endpoints.Map("HelloA", context => WriteActionResult(context, actionA()));
    endpoints.Map("HelloB", context => WriteActionResult(context, actionB()));
});

Then in the browser, the response to request http://localhost:59716/HelloA :

An error occured when executing context.RequestServices.GetRequiredService<IActionResultExecutor<TResult>>(); in WriteActionResult<TResult>() :

InvalidOperationException: No service for type 'Microsoft.AspNetCore.Mvc.Infrastructure.IActionResultExecutor`1[Microsoft.AspNetCore.Mvc.IActionResult]' has been registered.

But the response to request http://localhost:59716/HelloB is normal:

I have read the source of how IActionResultExecutor is registered here in services.AddMvcCore() .

But I failed to understand why it failed to handle a delegate instance of type delegate IActionResult RouteHandler() while it succeeded for delegate ContentResult ContentRouteHandler() .

My question is how to solve this problem?

Answer 1

Use reflection

    public static Task WriteActionResult<TResult>(HttpContext context, TResult result)
    where TResult : class, IActionResult
{
    var resultType = result.GetType();
    var executorType = typeof(IActionResultExecutor<>).MakeGenericType(resultType);
    var executor = context.RequestServices.GetRequiredService(executorType);
    var routeData = context.GetRouteData() ?? new RouteData();
    var actionContext = new ActionContext(context, routeData, new ActionDescriptor());
    var method = executorType.GetMethod(nameof(IActionResultExecutor<IActionResult>.ExecuteAsync)) ?? throw new MissingMethodException($"Missing 'ExecuteAsync' method");
    return (Task)method.Invoke(executor, new[] {actionContext, Convert.ChangeType(result, resultType)});
}

Answer 2

For the first route:

endpoints.Map("HelloA", context => WriteActionResult(context, actionA()));

You are calling WriteActionResult<IActionResult>(context, actionA()) , in which causes IActionResultExecutor<IActionResult> to be resolved.

There is no IActionResultExecutor<IActionResult> by default registered. So the exception is thrown.

However, there is IActionResultExecutor<ContentResult> by default registerd. So when you call the second route:

endpoints.Map("HelloB", context => WriteActionResult<ContentResult>(context, actionB()));

it just works.

To fix first route, try this:

endpoints.MapGet(
    "HelloA",
    context => WriteActionResult(context, (ContentResult)actionA())
);

Or you could fix RouteHandler like:

public delegate T RouteHandler<out T>() where T : IActionResult;

Answer 3

The solution is to use dynamic :

Before:

RouteHandler actionA =
    () => new ContentResult {Content = "Hello A!"};

After:

Func<dynamic> actionA =
    () => new ContentResult {Content = "Hello A!"};

Result:

At runtime, Func<dynamic> will be resolved to the actual type Func<ContentResult> , so the return type of actionA() will be ContentResult and can be consumed later by IActionResultExecutor<ContentResult> , which has already been registered.