IResultFilter没有一致的写入响应

Question

我有以下代码：

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {            
        services.AddMvc().AddMvcOptions(options =>
        {
            options.Filters.Add(new MessageAttribute("This is the Globally-Scoped Filter"));
        });
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseMvcWithDefaultRoute();
    }
}

public class MessageAttribute : ResultFilterAttribute
{
    private string message;
    public MessageAttribute(string msg)
    {
        message = msg;
    }

    public override void OnResultExecuting(ResultExecutingContext context)
    {
        WriteMessage(context, $"<div>Before Result:{message}</div>");
    }

    public override void OnResultExecuted(ResultExecutedContext context)
    {
        WriteMessage(context, $"<div>After Result:{message}</div>");
    }

    private void WriteMessage(FilterContext context, string msg)
    {
        var bytes = Encoding.ASCII.GetBytes($"<div>{msg}</div>");
        context.HttpContext.Response.Body.Write(bytes, 0, bytes.Length);
    }
}

[Message("This is the Controller-Scoped Filter", Order = 10)]
public class HomeController : Controller
{
    [Message("This is the First Action-Scoped Filter", Order = 1)]
    [Message("This is the Second Action-Scoped Filter", Order = -1)]
    public ViewResult Index()
    {
        return View("Message", $"This is the {nameof(Index)} action on the {nameof(HomeController)}");
    }
}

当我点击Index()动作方法时。我得到了以下三个输出中的一个：

1)

Before Result:This is the Second Action-Scoped Filter
Before Result:This is the Globally-Scoped Filter
Before Result:This is the First Action-Scoped Filter
Before Result:This is the Controller-Scoped Filter
After Result:This is the Controller-Scoped Filter
After Result:This is the First Action-Scoped Filter
After Result:This is the Globally-Scoped Filter
After Result:This is the Second Action-Scoped Filter

1. ​

Before Result:This is the Second Action-Scoped Filter
Before Result:This is the Globally-Scoped Filter
Before Result:This is the First Action-Scoped Filter
Before Result:This is the Controller-Scoped Filter

1. ​

空白页

当页面加载时，通常是2)，但有时是3)。刷新后，它有时会给我1)，但是不断刷新页面会给我带来随机的结果，更倾向于2)和3)。

如果我在WriteMessage()中放置一个断点，让它中断，然后继续执行，它就会给出预期的输出。

为什么会发生这种不一致的行为呢？

Answer 1

Adam是Pro ASP.NET Core 2的作者，我从那里获得了代码片段：

在

2.0中，直到管道中的所有组件都处理了请求之后，才会编写响应。在2.1中，这种情况发生了变化，以便在生成内容后立即编写响应。当您尝试添加到主体时，ASP.NET核心会感到困惑，因为它需要更新内容长度标题，但不能更新，因为响应已经被写入了。

出现不一致的结果是因为您已经创建了一个竞赛条件。有时，在分配线程来写出响应之前，过滤器会修改响应，有时则不会。您可以使用书中指定的ASP.NET核心版本，也可以更新您的代码，这样它就不会在action方法呈现其视图后尝试修改响应。