How to get RequestCachePolicy to respect max-age

问题

I am confused about how time-based cache policies work when using HttpWebRequest.

I am calling a GET method of a WebAPI that returns JSON content together with a Cache-Control header similar to:

Cache-Control: public, max-age=60

The intention is that the content should be considered to be stale after max-age seconds.

I am calling this API using HttpWebRequest, and want subsequent requests to be served from the local cache for max-age seconds, then to be retrieved from the server when the content expires.

I've tried different combinations with the results listed below.

1. Do not specify a RequestCachePolicy. In this case, as expected, all requests go to the server.

2. Specify a default cache policy:

   var policy = new RequestCachePolicy(RequestCacheLevel.Default);
   HttpWebRequest.DefaultCachePolicy = policy;
   

   In this case, all requests still go to the server. I was expecting requests to be served from the cache for the next max-age seconds.

3. Specify a cache policy CacheIfAvailable:

   var policy = new RequestCachePolicy(RequestCacheLevel.CacheIfAvailable);
   HttpWebRequest.DefaultCachePolicy = policy;
   

   In this case, all requests are served from the local cache, even if the content is stale (i.e. max-age seconds has passed).

Is it possible to achieve what I want, and if so, how?

回答1:

It seems that it's necessary for the server to add an Expires header to the response, in addition to (or instead of?) the Cache-Control header. The poster of that answer deserves the bounty.

When an Expires header is present, and RequestCacheLevel.Default is used, expiry behaves as expected in a .NET Framework application: the request is served from the local cache until the expiry time is reached.

However in a .NET Core application, using the same code, all requests are sent to the server and the local cache isn't used :( This is separate to the subject of my original question so I've asked a new question.

来源：https://stackoverflow.com/questions/55693462/how-to-get-requestcachepolicy-to-respect-max-age