C# How to use FirstOrDefault with an array while unable to have System.Linq in the namespace?

Question

The code provided here works when I test it in the IDE, but the software itself which will be using this code doesn't give me the possibility declare using System.Linq. I'm stuck trying to figure out how to use this else I'll have to go back to a solution I'd rather not use because it has a lower accuracy rate.

The issue is here

var stringMatch = sArray.FirstOrDefault(titleID.Contains);

I'm not sure how to properly provide the references. I think it should be something along the ways of System.Linq.Enumerable... but it's the first time I'm dealing with such an issue so any help much appreciated.

        string TakeEndPeriod = "";
        string NewEndPeriod = "";
        string FindEndSpace = "";
        string GetEndPeriod = "";
        string titleID = "document for the period ended December 31 2014";

        string s1 = "ended ";
        string s2 = "Ended ";
        string s3 = "Ending ";
        string[] sArray = new [] { s1, s2, s3};


             var stringMatch = sArray.FirstOrDefault(titleID.Contains);
            if (stringMatch != null)
            {
                TakeEndPeriod = titleID.Substring(titleID.LastIndexOf(stringMatch));
                FindEndSpace = TakeEndPeriod.Substring(TakeEndPeriod.IndexOf(" "));
                GetEndPeriod = FindEndSpace.Substring(1);

               string[] formatArray = new[] { "dd MMMM yyyy", "MMMM dd yyyy" };

                DateTime ModEndPeriod;
                if (!DateTime.TryParseExact(GetEndPeriod,
                                            formatArray,
                                            System.Globalization.CultureInfo.InvariantCulture,
                                            System.Globalization.DateTimeStyles.None,
                                            out ModEndPeriod))
                {
                        //parsing failed

                }

                NewEndPeriod = ModEndPeriod.ToString("yyyy-MM-ddT00:00:00Z");

EDIT:

error message I'm getting:

'System.Array' does not contain a definition for 'FirstOrDefault' and no extension
 method 'FirstOrDefault' accepting a first argument of type 'System.Array' could be
 found (are you missing a using directive or an assembly reference?)

EDIT:

This is the solution I got from the dev support managing the software:

String Title = "document for the period Ending December 31 2014";
            Match M = Regex.Match(Title, ".*?(?i)(ended|ending)(.*)");//Case insensitive search for "ended" and "ending"
            if (M.Groups.Count == 3)
            {
                //Match OK
                DateTime DT = DateTime.Parse(M.Groups[2].Value);
                //The variable DT will now have the parsed date.
                Console.WriteLine(DT.ToString("yyyyMMdd"));
            }

Answer 1

doesn't give me the possibility to declare 'using System.Linq'.

Since Enumerable.FirstOrDefault is an extension method you can also use it's fully qualified class-name System.Linq.Enumerable:

var stringMatch = System.Linq.Enumerable.FirstOrDefault(sArray, s => titleID.Contains(s));

or even shorter:

var stringMatch = System.Linq.Enumerable.FirstOrDefault(sArray, titleID.Contains);

Note that an extension method is simply a static method, that's why that works.

Demo