if (arry.Contains(str))

using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;namespace ConsoleApplication5
 {
     class Program
     {
         static void Main(string[] args)
         {
             String[] arry =new String[]{"abcdef","ghijklmn"};
             String str="abcd";
             if (arry.Contains(str))
             {
                 Console.WriteLine("A");
             } 
             else
             {
                 Console.WriteLine("B");//B
             }
         }
     }
 }