ASP.NET cache issue

Question

I'm caching MyData to avoid unnecessary calls to web service but it seems there is some problem. It is possible that caching is working correctly but I failed to figure out what causes problems. This is how I cache my data. I use static cache helper. You can google "cache helper c#" if you need that piece of code.

     private static object locker = new object();
    private static List<SomeDataDto> myData;
    private static List<SomeDataDto> MyData{
        get{
            if (CacheHelper.Exists("MyData") == false){
                lock (locker){

                    if (CacheHelper.Exists("MyData") == true){
                        myData=(List<SomeDataDto>)CacheHelper.GetCacheObject("MyData");
                    }
                    else
                    if (CacheHelper.Exists("MyData") == false){
                        var myData = GetSomethingFromDatabase("en", true);
                    CacheHelper.Add(myData, "MyData", 360);
                }
            }
            }
            return (List<SomeDataDto>)CacheHelper.GetCacheObject("MyData");
        }
    }

I'm calling this method on my page and this method throws error (sometimes!)

  public SomeObjectToReturn GetOneItem(string language, string id)
    {

        return MyData.Where(x => x.Language == language.ToUpper()).SelectMany(x => x.Something).SelectMany(x => x.SomethingElse).Where(x => x.ID == id).FirstOrDefault();
    }

I call this method 15 times for 15 different id-s but it works for 10 items or 8 items or something like that. If I edit config to remove cache some other id-s are not working. So:

1. First time it works id=6
2. Second time it doesn't work id=6

Is it cache corrupted? Web Service does not return all elements in list? (too many of them?)

Answer 1

You have to include your return (List)CacheHelper.GetCacheObject("MyData"); in your lock like:

    get{
        lock (locker){
           if (CacheHelper.Exists("MyData") == false){

                if (CacheHelper.Exists("MyData") == true){
                    myData=(List<SomeDataDto>)CacheHelper.GetCacheObject("MyData");
                }
                else
                if (CacheHelper.Exists("MyData") == false){
                    var myData = GetSomethingFromDatabase("en", true);
                CacheHelper.Add(myData, "MyData", 360);
            }
          }
         return (List<SomeDataDto>)CacheHelper.GetCacheObject("MyData");
       }
    }

If you read while you are writing or deleting the behavior could be different as you expect. The List object is not thread safe.