c# - 将响应 header 添加到 ASP.NET Core 中间件

Question

我想像这样向我的 ASP.NET Core WebApi 添加一个处理时间中间件

public class ProcessingTimeMiddleware  
{
    private readonly RequestDelegate _next;

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

    public async Task Invoke(HttpContext context)
    {
        var watch = new Stopwatch();
        watch.Start();

        await _next(context);

        context.Response.Headers.Add("X-Processing-Time-Milliseconds", new[] { watch.ElapsedMilliseconds.ToString() });
    }
}

但是这样做会抛出异常

 System.InvalidOperationException: Headers are readonly, reponse has already started.

如何向响应添加 header ？

Answer 1

没关系，代码在这里

    public async Task Invoke(HttpContext context)
    {
        var watch = new Stopwatch();
        watch.Start();

        //To add Headers AFTER everything you need to do this
        context.Response.OnStarting(state => {
            var httpContext = (HttpContext)state;
            httpContext.Response.Headers.Add("X-Response-Time-Milliseconds", new[] { watch.ElapsedMilliseconds.ToString() });

            return Task.CompletedTask;
        }, context);

        await _next(context);
    }