Does Index of Array Exist
What exactly do you mean by "is a valid element"? You could just do:
if (array.Length >= 26)
which would tell you whether 25 is a valid index into the array or not (assuming a 0 lower bound).
If you need to know whether it's non-null or not, just use:
if (array[25] != null)
(or a combination of the two).
If these don't help, please give a more precise meaning of "valid" for your problem.
You can use LINQ to achieve that too:
var exists = array.ElementAtOrDefault(index) != null;
Test the length
int index = 25;
if(index < array.Length)
{
//it exists
}