use of the Operator += and append from "StringBuilder class

//stringbuler
//csc string_plusequaloperator.cs
using System;
using System.Text;
namespace ex_stringbuild
{
class plusequal
{
public void mystrbuilder(string f, string l, int id)
{
string output= "";
output +=f;
output +=" ";
output +=l;
output +=" ";
output +=id;

Console.WriteLine("string joined : " + output);
System.Text.StringBuilder str = new System.Text.StringBuilder();
Console.WriteLine("Hand 0ver to String Builder append : " );
str.Append (output);
Console.WriteLine("String Accepted : " + str.Capacity);
Console.WriteLine("String Length : " + str.Length);
}
}

class test
{
static void Main(string[] args)
{
plusequal ss = new plusequal();

ss.mystrbuilder("Manas","Mukherjee", 12345);
Console.ReadLine();

}
}


}