C# program to binary Search int

 using System;  
 using System.Collections.Generic;  
 using System.Linq;  
 using System.Text;  
 using System.Threading.Tasks;  
 public class Main  
 {  
   public static int binarySearch(int st, Func<int, bool> func)  
   {  
     int ret = 0;  
     subBinarySearch(0, st, out ret, func);  
     return ret;  
   }  
   static void subBinarySearch(int begin, int end, out int idx, Func<int, bool> func)  
   {  
     if (begin == end || begin == end - 1)  
     {  
       idx = end;  
     }  
     else  
     {  
       int mid = (begin + end) / 2;  
       if (func(mid))  
       {  
         subBinarySearch(mid, end, out idx, func);//tail optimas, of couse  
       }  
       else  
       {  
         subBinarySearch(begin, mid, out idx, func);  
       }  
     }  
   }  
 }