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c#refoutreference-type

Why passing string to a method behaves unlike List<object> if they are both reference type?

I was reading a code that made me stop at some line:

List<object> props = new List<object>();
DoWork(param1, param2, props);
//props.Count grew up

I was thinking that changing a variable outside its scope requires passing it as out or ref, but then I realized that unless the DoWork method changes the props reference like:

props = new List<object>();

the reference will be point to the same location. So there is no need to use ref here.

and then I created a method for string type:

static void ChangeMe(string str)
{
    str = "W77";
}
public static void Main(string[] args)
{
    string str = "p1";
    ChangeMe(str);
    //str is still "p1"
}

If the behavior motivated the List to be changed outside its scope is that it's reference type, why string doesn't change if it's not reallocated in the callee method and it's a reference type like List<object>?

Solution

  • Reason is because string is an immutable type in C# and therefore whenever you "change" it what actually you are doing is creating a new string with the new value and referencing to it.

    When calling the ChangeMe a new reference to the same string is created. Then in the method you are referencing the new reference to a completely new string, which does not influence the previous string defined in Main.