string[] arrTopics = {\"Health\", \"Science\", \"Politics\"};
I have an if statement like:
 if (arrTopics.Count() != null)
<         
        
Your if statement doesn't look right. I assume. You either meant to
if(arrTopics?.Count() != null)
OR
if (arrTopics.Any())
OR
if (arrTopics != null)
Explanation:
sequence.Count() returns int which is never null. Adding the question mark ? returns Nullable instead of int. Also Count() expects the sequence to be instantiated. using Count() method on a null sequence results in ArgumentNullException; so you always need to check against null reference, before going for Count(), Any() extension methods, or use the question mark.