which operator is used to concatenate strings in c#?

static void Main(string[] args)
{
    // To run this program, provide a command line string.
    // In Visual Studio, see Project > Properties > Debug.
    string userName = args[0];
    string date = DateTime.Today.ToShortDateString();

    // Use the + and += operators for one-time concatenations.
    string str = "Hello " + userName + ". Today is " + date + ".";
    System.Console.WriteLine(str);

    str += " How are you today?";
    System.Console.WriteLine(str);

    // Keep the console window open in debug mode.
    Console.WriteLine("Press any key to exit.");
    Console.ReadKey();
}

// Example output: 
//  Hello Alexander. Today is 1/22/2008.
//  Hello Alexander. Today is 1/22/2008. How are you today?
//  Press any key to exit.
//