asp.net-core – 如何检索当前响应体长度？

这是一个中间件示例.在任何生成中间件(如useMVC或useStaticFiles)的响应之前,应将其添加到管道(startup.cs)中.

public class ContentLengthMiddleware
{
    RequestDelegate _next;

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

    public async Task Invoke(HttpContext context)
    {
        using (var buffer = new MemoryStream())
        {
            var request = context.Request;
            var response = context.Response;

            var bodyStream = response.Body;
            response.Body = buffer;

            await _next(context);
            Debug.WriteLine($"{request.Path} ({response.ContentType}) Content-Length: {response.ContentLength ?? buffer.Length}");
            buffer.Position = 0;
            await buffer.CopyToAsync(bodyStream);
        }
    }
}

由于在返回静态文件(png,js等)时我无法理解的原因,响应体将为空但是ContentLength设置为我使用response.ContentLength的原因？ buffer.Length.

(注意mod：抱歉两个问题的重复答案.另一个答案是错误发布,打开了太多标签.我删除了它并在此处重新发布了答案).