You can use extension methods and Lambda expressions to achieve this.
If you need the Index of the first element that contains the substring in the array elements, you can do this...
int index = Array.FindIndex(arrayStrings, s => s.StartsWith(lineVar, StringComparison.OrdinalIgnoreCase)) // Use 'Ordinal' if you want to use the Case Checking.
If you need the element's value that contains the substring, just use the array with the index you just got, like this...
string fullString = arrayStrings[index];
Note: The above code will find the first occurrence of the match. Similary, you
can use Array.FindLastIndex() method if you want the last element in the array
that contains the substring.
You will need to convert the array to a List<string> and then use the ForEach extension method along with Lambda expressions to get each element that contains the substring.