检查空的HttpContext.Current.Cache值

Question

In the below snippet, I am trying to assign the cache value if the cache value does not already exist. I get an Object_reference_not_set_to_an_instance_of_an_object error when running the following. What am I missing?

if (string.IsNullOrEmpty(HttpContext.Current.Cache[Key].ToString()))
                HttpContext.Current.Cache[Key] = data;

I looked around on SO but couldnt find something similar. Maybe I am just not wording my problem correctly.

Answer 1

HttpContext.Current could be null. HttpContext.Current.Cache[Key] could be null.

If either of those are null, it would result in the error you are getting.

Answer 2

You are getting the NullReferenceException because you are trying to call ToString() on a null instance.

You have to check if HttpContext.Current.Cache[Key] is null before calling ToString()

if (HttpContext.Current.Cache[Key] == null)
{
   HttpContext.Current.Cache[Key] = data;
}

Answer 3

You should check for null on HttpContext.Current and on HttpContext.Current.Cache[Key] , both of which could be null. Here's a possible solution, as long as you're okay with not setting the cache key if HttpContext.Current is null.

if (HttpContext.Current != null &&
    (HttpContext.Current.Cache[Key] == null || string.IsNullOrEmpty(HttpContext.Current.Cache[Key].ToString()))
{
     HttpContext.Current.Cache[Key] = data;
}

Answer 4

I just changed the 'get value, convert to string, compare' logic to just get the value and see if it's null right up front. Silly me.

if (HttpContext.Current.Cache[Key] == null)
       HttpContext.Current.Cache[Key] = data;

The " Object_reference_not_set_to_an_instance_of_an_object error" is actually essentially the 'null' value I was looking for...

Answer 5

If they key isn't present, then this would return null :

HttpContext.Current.Cache[Key]

You're then blindly calling ToString() on the value from the cache, causing the exception.

You need to assign the value from the cache to a temporary variable and test it for null before calling ToString() .