c# - Add Response Headers to ASP.NET Core Middleware

Question

I want to add a processing time middleware to my ASP.NET Core WebApi like this

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() });
    }
}

But doing this throws an exception saying

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

How can I add headers to the response?

Answer 1

Never mind, the code is here

    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);
    }