No conversion available between HelloWorldComponent and System.Func`2[System.Collections.Generic.IDictionary`2 // Parameter name: signature

问题

I am working through Scott Allen's MVC 5 Fundamentals course on Pluralsight

I get an error at "using (WebApp.Start(uri)) " in the code below.

The error is

An unhandled exception of type 'System.ArgumentException' occurred in Microsoft.Owin.dll
  System.ArgumentException was unhandled
  HResult=-2147024809
  Message=No conversion available between ConsoleApplication1.HelloWorldComponent and System.Func`2[System.Collections.Generic.IDictionary`2[System.String,System.Object],System.Threading.Tasks.Task].
Parameter name: signature
  Source=Microsoft.Owin
  ParamName=signature
  StackTrace:
       at Microsoft.Owin.Builder.AppBuilder.Convert(Type signature, Object app)
       at Microsoft.Owin.Builder.AppBuilder.BuildInternal(Type signature)
       at Microsoft.Owin.Builder.AppBuilder.Build(Type returnType)
       at Microsoft.Owin.Hosting.ServerFactory.ServerFactoryAdapter.Create(IAppBuilder builder)
       at Microsoft.Owin.Hosting.Engine.HostingEngine.StartServer(StartContext context)
       at Microsoft.Owin.Hosting.Engine.HostingEngine.Start(StartContext context)
       at Microsoft.Owin.Hosting.Starter.DirectHostingStarter.Start(StartOptions options)
       at Microsoft.Owin.Hosting.Starter.HostingStarter.Start(StartOptions options)
       at Microsoft.Owin.Hosting.WebApp.StartImplementation(IServiceProvider services, StartOptions options)
       at Microsoft.Owin.Hosting.WebApp.Start(StartOptions options)
       at Microsoft.Owin.Hosting.WebApp.Start[TStartup](StartOptions options)
       at Microsoft.Owin.Hosting.WebApp.Start[TStartup](String url)
       at ConsoleApplication1.Program.Main(String[] args) in e:\EShared\Dev2015\WebAppScottAllen\ConsoleApplication1\ConsoleApplication1\Program.cs:line 16
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

The code is

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;
using Microsoft.Owin.Hosting;
using Owin;
namespace ConsoleApplication1
{
    using AppFunc = Func<IDictionary<string, object>, Task>;
    class Program
    {
        static void Main(string[] args)
        {
            string uri = "http://localhost:8080";
            using (WebApp.Start<Startup>(uri))   // Katana Please start, using the configuration from the Startup class and listening on the port given by the uri
            {
                Console.WriteLine("Started!");
                Console.ReadKey();
                Console.WriteLine("Stopping!");
            }
        }
    }

    public class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            app.Use<HelloWorldComponent>();
        }
    }

    public class HelloWorldComponent
    {
        AppFunc _next;
        public HelloWorldComponent(AppFunc next)
        {
            _next = next;
        }

        // Katana uses reflection to find this Invoke function that matches the AppFunc signature
        public Task Invoke(IDictionary<string, object> environment)
        { 
            var response = environment["owin.ResponseBody"] as Stream;

            using (var writer = new StreamWriter(response))
            {
                return writer.WriteAsync("Hello");
            }
        }
    }
}

packages.config is

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Microsoft.Owin" version="3.0.1" targetFramework="net451" />
  <package id="Microsoft.Owin.Diagnostics" version="3.0.1" targetFramework="net451" />
  <package id="Microsoft.Owin.Host.HttpListener" version="3.0.1" targetFramework="net451" />
  <package id="Microsoft.Owin.Host.SystemWeb" version="3.0.1" targetFramework="net451" />
  <package id="Microsoft.Owin.Hosting" version="3.0.1" targetFramework="net451" />
  <package id="Owin" version="1.0" targetFramework="net451" />
</packages>

I wonder did not use to get this message, so I wonder what could have changed

回答1:

There is a new way to write our middlewares components, which looks like this:

public class HelloWorldComponent : OwinMiddleware
{
    public HelloWorldComponent(OwinMiddleware next) : base(next) { }

    public override Task Invoke(IOwinContext context)
    {
        return context.Response.WriteAsync("Hello, World!");
    }
}

Specifically, the constructor must accept an OwinMiddleware reference as its first parameter, otherwise you get an error because the ctor signature does not match what is expected by the current Owin implementation.

Further, consider the following parameterized usage:

    var param1 = "Hello, World!";
    appBuilder.Use<HelloWorldComponent>(param1)

To properly support this you will want a modified constructor signature:

public class HelloWorldComponent : OwinMiddleware
{
    public HelloWorldComponent(OwinMiddleware next) : base(next) { }

    public override Task Invoke(IOwinContext context, string param1)
    {
        return context.Response.WriteAsync(param1);
    }
}

Thus, allowing us to parameterize our middleware via Use()'s params array.

回答2:

Inheriting from OwinMiddleware limits you to the Katana implementation of OWIN.
Creating a OwinContext from the passed environment should work for you

class HelloWorldComponent 
    {
        private readonly AppFunc _next;

        public HelloWorldComponent (AppFunc next)
        {
            _next = next;
        }

        public async Task Invoke(IDictionary<string, object> environment)
        {

            var ctx = new OwinContext(environment);
            await ctx.Response.WriteAsync("Hello World");
            await _next(environment);
        }
    }

来源：https://stackoverflow.com/questions/29803698/no-conversion-available-between-helloworldcomponent-and-system-func2system-col