string类型在参数传递中如何作为引用，如何在函数外修改函数内的string类型

class StringExc
{
public static void Main()
{
string s1 = "original string";
Function(ref s1);
}

public void Function(ref string s1)
{
s1 = "sssss";
}
}

class StringExc
{
object s2;   //此处是object，如果是string类型的，则改变s2的值不影响s1的值
public static void Main()
{
string s1 = "original string";
s2 = s1;
Function();
}

public void Function()
{
s2 = "sssss";
}
}