Question

Why does the original string remain unchanged in example 1 and the original instance not get nulled in example 2, when the string property of an object instance is changed in example 3?

    class A { public string property = "class a"; }

    static void Main(string[] args)
    {

        //Example1
        string var = "something";
        string[] array = new string[] { var };
        array[0] += " again";

        // var is unchanged, contents in array slot zero are changed

        //Example2
        A instance = new A();
        object[] array2 = new object[] { instance };
        array2[0] = null;

        // instance is also unchanged, contents in array slot zero are now null

        //Example3
        A anotherInstance = new A();
        object[] array3 = new object[] { anotherInstance };
        (array3[0] as A).property = " else";

        // the *propery* of anotherInstance changes as expected

        return;
    } 

Answer 1

When you assign a value in an array, you create a copy of the original value. You’re not creating a copy of the string itself, but a copy of the value of the “var” variable – which is a string reference. After that point of time, there’s no relationship between the variable and the array element – they happen to have the same value immediately afterwards, but they’re independent. Changing the array element to make it refer to a different object doesn’t change the value of the variable, or vice versa.

In particular, this line:

array[0] += " again";

does not change the contents of the existing string. Instead, it creates a new string and assigns that reference to array[0].

Now in your “Example3” you aren’t changing the value of the array element itself – you’re changing the contents of the object that both anotherInstance and array3[0] refer to. It’s like two people knowing about the same house – if one person paints the door red, the other person will see the red door too.

This all has very little to do with arrays – you’d see the same effects with individual variables too. See my article on reference and value types for further information.