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(); } }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：
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: :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