没有注册类型服务(No service for type has been registered)

 我正在尝试实现ASP.NET Core中间件，这是我在项目中的完整代码： 
 
public class HostMiddleware : IMiddleware
{
    public int Count { get; set; }

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        if (context.Request.Query.ContainsKey("hello"))
        {
            await context.Response.WriteAsync($"Hello World: {++Count}");
        }
        else
        {
            await next.Invoke(context);
        }
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, IServiceProvider provider)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseMiddleware<HostMiddleware>();

        app.Run(async context =>
        {
            context.Response.StatusCode = 400;
            await context.Response.WriteAsync("Bad request.");
        });
    }
 
 但是，当我运行此服务器时，我收到以下错误： 
 
 
 
 InvalidOperationException：未注册“WebApplication4.HostMiddleware”类型的服务。 
 
 
  
 
 为什么我会收到此错误？ 如果我在项目中不使用依赖注入，为什么我的中间件需要注册任何服务？ 
 
 更新： 
 
 出于某种原因，当我停止使用IMiddleware ，将InvokeAsync重命名为Invoke并以下列方式实现我的中间件时，不会发生此错误： 
 
public class WmsHostMiddleware 
{
    private readonly RequestDelegate _next;

    public int Count { get; set; }

    public WmsHostMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        if (context.Request.Query.ContainsKey("hello"))
        {
            await context.Response.WriteAsync($"Hello World: {++Count}");
        }
        else
        {
            await _next.Invoke(context);
        }
    }
}
 
 问题仍然存在 - 为什么会发生这种情况？ 有什么不同？ 当我使用IMiddleware时，为什么需要注册服务。 

I am trying to implement ASP.NET Core middleware, and this is the whole code I have in my project:
 
public class HostMiddleware : IMiddleware
{
    public int Count { get; set; }

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        if (context.Request.Query.ContainsKey("hello"))
        {
            await context.Response.WriteAsync($"Hello World: {++Count}");
        }
        else
        {
            await next.Invoke(context);
        }
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, IServiceProvider provider)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseMiddleware<HostMiddleware>();

        app.Run(async context =>
        {
            context.Response.StatusCode = 400;
            await context.Response.WriteAsync("Bad request.");
        });
    }
 
However, when I run this server I get the following error:
 
 
 
InvalidOperationException: No service for type 'WebApplication4.HostMiddleware' has been registered.
 
 
 
Why do I get this error? Why would my middleware need to register any services if I don't use dependency injection in my project? 
 
Update:
 
For some reason, this error does not occur when I stop using IMiddleware, rename InvokeAsync to Invoke and implement my middleware in the following way:
 
public class WmsHostMiddleware 
{
    private readonly RequestDelegate _next;

    public int Count { get; set; }

    public WmsHostMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        if (context.Request.Query.ContainsKey("hello"))
        {
            await context.Response.WriteAsync($"Hello World: {++Count}");
        }
        else
        {
            await _next.Invoke(context);
        }
    }
}
 
The question is still open - why does this happen? What is the difference? Why do I need to register services when I use IMiddleware.

原文：https://stackoverflow.com/questions/48394756