string strA="abcde";
string strB="abc";
string strC="de";
Console.WriteLine(strA == (strB+strC));//true
Console.WriteLine((object)strA == (object)(strB+strC));//false

string strA="abcde";
strA="aaaaa";

string strA="abcde";//创建一个内容为abcde的string对象，然后将该对象的引用赋给strA

char[] arr={'a','b','c','d','e'};
string strA=new string(arr);//这里只列举一种

static void Main(string[] args)
{
  string strA = "abcde";
  Deal(strA);
  Console.WriteLine(strA);
  Console.ReadLine();
}
static void Deal(string str)
{
  str = str.Substring(0, 2);
}

static void Main(string[] args)
{
   string strA = "abcde";
   Deal(strA);
   Console.WriteLine(ref strA);
   Console.ReadLine();
}
static void Deal(ref string str)
{
   str = str.Substring(0, 2);
}

static void Main(string[] args)
{
  string strA = "1";
  string strB = string.Empty;
  string strC = null;

  Console.WriteLine(int.Parse(strA));//正确
  Console.WriteLine(int.Parse(strB));//输入字符串的格式不正确
  Console.WriteLine(strC.ToString());//未将对象引用设置到对象的实例。
  Console.ReadLine();
}

string strA="abc"
for(int i=0;i<10000;i++)
{
  strA+="abc";
}
Consolse.WriteLine(strA);

StringBuilder strA=new StringBuilder();
for(int i=0;i<10000;i++)
{
strA.Append("abc");
}
Consolse.WriteLine(strA.ToString());