IHttpContextAccessor提供对上下文的访问并使用实现.感谢@ mj1313在此论坛中指出答案.，

IHttpContextAccessor provides access to context and use the implementation. Thanks to @mj1313 for pointing out the answer in this forum., Link

public class HomeController : Controller
{
    private readonly IHttpContextAccessor _httpContextAccessor;

    public HomeController(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }
 

    public async Task<IActionResult> Contact()
    {
       ..

       var resp = _httpContextAccessor.HttpContext.Response;
       resp.Headers.Add("Content-Type", "text/event-stream");

       // Verify that the client is connected.
       if (HttpContext.RequestAborted.IsCancellationRequested == false)
            {
                ..                   
                await resp.Body.WriteAsync(buffer, 0, length); 
                //or await resp.WriteAsync("hello"); 
                               
                resp.Body.Flush();

                ..
            }
       ..
    
    }
}