String object is really by reference? [duplicate]

Answer 1

The MSDN page can help explain why it is a reference type.

Because String is a collection of chars and inherits from various Collection base classes, it is a class, and therefore a reference type.

Answer 2

This has nothing to do with string being a value type or a reference type. The parameter of your changeMe method isn't marked ref, so if the method changes it, the caller will not see the changes.

Try that instead :

public static string changeMe(ref string param)
{
    param = "I have changed you!!!";

    return ""; // no return for test
}

Answer 3

The reason that Program.changeMe(str) does not cause str to be set to "I have changed you!!!" is that although string is a reference type, references are passed by value.

So param is a copy of the reference, it gets changed inside the scope of Program.changeMe(param) and then thrown away at the end of the method.

The reference str is only copied, not changed to reference "I have changed you!!!"

Answer 4

If you are planning on changing the value of an argument to a method you should really use the "ref" or "out" keywords. Otherwise you risk making confusing methods that may or may not do what you intend. As other have pointed out you are changing the value of the string reference and not the instance of the object. To see how this works with other classes I have written a quick example

static void Main(string[] args)
{
    var str = "String";
    var obj = new MyObject() { Value = "Object" };

    Console.WriteLine(str); //String
    Console.WriteLine(obj); //Object

    ChangeMe(str);
    ChangeMe(obj);

    Console.WriteLine(str); //String
    Console.WriteLine(obj); //Object

    ChangeMeInner(obj);
    // this is where it can get confusing!!!
    Console.WriteLine(obj); //Inner 

    ChangeMe(ref str);
    ChangeMe(ref obj); 

    Console.WriteLine(str); // Changed
    Console.WriteLine(obj); // Changed

    Console.Read();
}
class MyObject
{
    public string Value { get; set; }
    public override string ToString()
    {
        return Value;
    }
}
static void ChangeMe(MyObject input)
{
    input = new MyObject() { Value = "Changed" };
}
static void ChangeMeInner(MyObject input)
{
    input.Value = "Inner";
}
static void ChangeMe(string input)
{
    input = "Changed";
}
static void ChangeMe(ref MyObject input)
{
    input = new MyObject() { Value = "Changed" };
}
static void ChangeMe(ref string input)
{
    input = "Changed";
}