C# で文字列配列を文字列に変換する

string.Join("", str); 

using System;

class HelloWorld
{
    static void Main()
    {
        string[] str = new string[2];
        str[0] = "Hello";
        str[1] = "World";
        string res = string.Join(".", str);
        Console.WriteLine($"{res}");
    }
}

Hello.World

string result = String.Concat(str);

using System;

public class HelloWorld {
  public static void Main(string[] args) {
    string[] str = new string[2];
    str[0] = "Hello ";
    str[1] = "World";
    string res = String.Concat(str);
    Console.WriteLine($"{res}");
  }
}

Hello World

StringBuilder bdr = new StringBuilder();
        foreach (string value in array)
        {
            bdr.Append(value);
            bdr.Append('.');
        }

using System;
using System.Text;

class HelloWorld
{
    static void Main()
    {
    string[] array = new string[] { "Hello","World"};
    StringBuilder bdr = new StringBuilder();
        foreach (string value in array)
        {
            bdr.Append(value);
            bdr.Append('.');
        }
        string res = bdr.ToString();
        Console.WriteLine($" {res}");
    }
}

Hello.World.