ASP.NET Core 异常 Headers are read-only, response has already started

小渣渣

需求：ASP.NET Core 在中间件修改响应头 Server 信息，报错：System.InvalidOperationException: Headers are read-only, response has already started.

错误写法：

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报错如下：

fail: Microsoft.AspNetCore.Server.Kestrel[13]
      Connection id "0HN519819L24M", Request id "0HN519819L24M:00000001": An unhandled exception was thrown by the application.
      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.HttpResponseHeaders.Microsoft.AspNetCore.Http.IHeaderDictionary.set_Server(StringValues value)

异常原因是因为当 ASP.NET Core 已经开始响应了，Headers 响应头则不允许再次被修改了，也就是当开始响应后 Headers 将变成只读状态。

解决方案如下：

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如下图：