Read Asp.Net Core Response body in ActionFilterAttribute

If you're logging for json result/ view result , you don't need to read the whole response stream. Simply serialize the context.Result:

public class MyFilterAttribute : ActionFilterAttribute
{
    private ILogger<MyFilterAttribute> logger;

    public MyFilterAttribute(ILogger<MyFilterAttribute> logger){
        this.logger = logger;
    }
    public override void OnActionExecuted(ActionExecutedContext context)
    {
        var result = context.Result;
        if (result is JsonResult json)
        {
            var x = json.Value;
            var status = json.StatusCode;
            this.logger.LogInformation(JsonConvert.SerializeObject(x));
        }
        if(result is ViewResult view){
            // I think it's better to log ViewData instead of the finally rendered template string
            var status = view.StatusCode;
            var x = view.ViewData;
            var name = view.ViewName;
            this.logger.LogInformation(JsonConvert.SerializeObject(x));
        }
        else{
            this.logger.LogInformation("...");
        }
    }

I know there is already an answer but I want to also add that the problem is the MVC pipeline has not populated the Response.Body when running an ActionFilter so you cannot access it. The Response.Body is populated by the MVC middleware.

If you want to read Response.Body then you need to create your own custom middleware to intercept the call when the Response object has been populated. There are numerous websites that can show you how to do this. One example is here.

As discussed in the other answer, if you want to do it in an ActionFilter you can use the context.Result to access the information.