为啥 Response.Write 后,View就不渲染了?

一:背景

1. 讲故事

前几天群里有一位朋友聊到,为什么我在 Action 中执行一句 Response.Write 之后,后续的 View 就不呈现了,如果脑子中没有画面,那就上测试代码:


    public class HomeController : Controller
    {
        public IActionResult Index()
        {
            Response.WriteAsync("hello world!");
            return View();
        }
    }

为啥 Response.Write 后,View就不渲染了?

结果还是挺有意思的,大家都知道,默认情况下会渲染 /Home/Index 对应的 view 页面,但这里被 Response.WriteAsync 插了一杠子,气的 view 都渲染不出来了,那接下来就来找一找 view 为啥这么生气?

二:寻找真相

1. 从 Logger 入手

相信很多人都在用 aspnetcore 中的 logger 记录日志,为什么要首选这个 logger 呢?因为它在 web框架 中是一等公民的存在,毕竟底层源码各处都嵌入着这玩意哈,随便找点代码:


internal abstract class ActionMethodExecutor
{
    private Task ResultNext<TFilter, TFilterAsync>(ref ResourceInvoker.State next, ref ResourceInvoker.Scope scope, [Nullable(2)] ref object state, ref bool isCompleted) where TFilter : class, IResultFilter where TFilterAsync : class, IAsyncResultFilter
    {
    	ResourceInvoker.ResultExecutingContextSealed resultExecutingContext3 = this._resultExecutingContext;
		this._diagnosticListener.BeforeOnResultExecuting(resultExecutingContext3, tfilter);
		this._logger.BeforeExecutingMethodOnFilter(filterType, "OnResultExecuting", tfilter);
		tfilter.OnResultExecuting(resultExecutingContext3);
		this._diagnosticListener.AfterOnResultExecuting(resultExecutingContext3, tfilter);
		this._logger.AfterExecutingMethodOnFilter(filterType, "OnResultExecuting", tfilter);
		if (this._resultExecutingContext.Cancel)
		{
			this._logger.ResultFilterShortCircuited(tfilter);
			this._resultExecutedContext = new ResourceInvoker.ResultExecutedContextSealed(resultExecutingContext3, this._filters, resultExecutingContext3.Result, this._instance)
			{
				Canceled = true
			};
			goto IL_39E;
		}
    }
}

而且大家想想,这种写法特别奇葩,我想底层框架中的 logger 定会有所反馈,接下来在启动程序的时候采用 WebApplication1 的模式启动,如下图:

为啥 Response.Write 后,View就不渲染了?

启动后,在控制台上可以看到一堆报错信息:


info: Microsoft.Hosting.Lifetime[0]
      Now listening on: http://localhost:5000
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: E:\net5\WebApplication1\WebApplication1
fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
System.InvalidOperationException: Headers are read-only, response has already started.
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.ThrowHeadersReadOnlyException()
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.Microsoft.AspNetCore.Http.IHeaderDictionary.set_Item(String key, StringValues value)
   at Microsoft.AspNetCore.Http.DefaultHttpResponse.set_ContentType(String value)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(ViewContext viewContext, String contentType, Nullable`1 statusCode)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(ActionContext actionContext, IView view, ViewDataDictionary viewData, ITempDataDictionary tempData, String contentType, Nullable`1 statusCode)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.ViewResultExecutor.ExecuteAsync(ActionContext context, ViewResult result)
   at Microsoft.AspNetCore.Mvc.ViewResult.ExecuteResultAsync(ActionContext context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeResultAsync>g__Logged|21_0(ResourceInvoker invoker, IActionResult result)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|29_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters()

异常信息非常明显:Headers are read-only, response has already started,大概就是说,header是只读的,response已是启动状态了,从调用堆栈的 ViewExecutor.ExecuteAsync 处可看出,代码准备渲染 view,在 set_ContentType 处遭遇异常,结束了后续渲染流程。

接下来一起看下,为什么会触发这个异常???

三: 调试源码寻找异常的原因

1. dnspy 调试

除了从异常堆栈中找到最早的异常代码处,这里还说一个小技巧,使用 ndspy 的 异常断点功能,在异常设置面板 定位 InvalidOperationException 异常即可。

为啥 Response.Write 后,View就不渲染了?

接下来就可以让程序跑起来,当异常抛出时会自动断下来。

为啥 Response.Write 后,View就不渲染了?

仔细看一下图中的文字标注,还是很好理解的,接下来继续追一下: response.ContentType = contentType2; 内部都做了什么。


		public override string ContentType
		{
			get
			{
				return this.Headers[HeaderNames.ContentType];
			}
			set
			{
				if (string.IsNullOrEmpty(value))
				{
					this.HttpResponseFeature.Headers.Remove(HeaderNames.ContentType);
					return;
				}
				this.HttpResponseFeature.Headers[HeaderNames.ContentType] = value;
			}
		}

可以看到 内部是给 this.HttpResponseFeature.Headers 赋值的,继续往下追:

为啥 Response.Write 后,View就不渲染了?

从图中可以看到,最后的 HttpHeader._isReadOnly =true 导致异常的发生,罪魁祸首哈,接下来研究下这句 HttpHeader._isReadOnly=true 是何时被赋值的。

2. _isReadOnly=true 何时发生

这个问题就简单多了,必定是 Response.WriteAsync("hello world!"); 造成了 _isReadOnly=true ,在 HttpHeader 下有一个 SetReadOnly 方法用于对 _isReadOnly 字段的封装,代码如下:


internal abstract class HttpHeaders 
{
    public void SetReadOnly()
    {
        this._isReadOnly = true;
    }
}        

接下来在该方法处下一个断点,继续调试,如下图:

为啥 Response.Write 后,View就不渲染了?

从图中可看到,原来 Response.WriteAsync("hello world!") 是可以* HttpHeaders的,后续任何再对 HttpHeader 的操作都是无效的。。。

其实大家也可以想一想,不同的response,肯定会有不同的 header,要想叠加的话这辈子都不可能的,只能让后面的报错,如下:


1. response:

HTTP/1.1 200 OK
Date: Mon, 19 Oct 2020 14:37:54 GMT
Server: Kestrel
Transfer-Encoding: chunked

c
hello world!


2. view:

HTTP/1.1 200 OK
Date: Mon, 19 Oct 2020 14:39:01 GMT
Content-Type: text/html; charset=utf-8
Server: Kestrel
Content-Length: 2239

四: 总结

这篇就是对群聊天过程中抛出问题的个人探究,一家之言,不过挺有意思,大家也可以多用用调试工具寻找问题,证明问题,纸上得来终觉浅,绝知此事要躬行,好了,希望本篇对您有帮助!

更多高质量干货:参见我的 GitHub: dotnetfly

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