Parsing from string to int

in my C# project I have an array type of String. the array gets numbers from type int.

int numCell;
string[] arr = new string[numCell];

then i use it in a function like this:

sum += int.Parse(arr[i]);

and it works!

but, when i dothe same thing in WPF it tells me:

Input string was not in a correct format.

what shouuld i do?