JoinString();

JoinString(); in C#
It utilized to connect the elements of an array, using the specified separator between each element. 
Example
using System;

namespace program

{
    class Program
    {
        static void Main(string[] args)
        {
            string[] s1 = { "A", "B", "C" };
            string s3 = string.Join("=", s1);
            Console.WriteLine(s3);  
            Console.ReadKey();
        }   
    }
 }

Output
A=B=C