C# Calling a Method, and variable scope

Question

Why is cards being changed below? Got me puzzled.. understand passing by ref which works ok.. but when passing an Array is doesn't do as I expect. Compiling under .NET3.5SP1

Many thanks

void btnCalculate_Click(object sender, EventArgs e)
{
    string[] cards = new string[3];
    cards[0] = "old0";
    cards[1] = "old1";
    cards[2] = "old2";
    int betResult = 5;
    int position = 5;
    clsRules myRules = new clsRules();
    myRules.DealHand(cards, betResult, ref position);  // why is this changing cards!
    for (int i = 0; i < 3; i++)
        textBox1.Text += cards[i] + "\r\n";  // these are all new[i] .. not expected!

    textBox1.Text += "betresult " + betResult.ToString() + "\r\n";  // this is 5 as expected
    textBox1.Text += "position " + position.ToString() + "\r\n"; // this is 6 as expected
}

public class clsRules
{
    public void DealHand(string[] cardsInternal, int betResultInternal, ref int position1Internal)
    {
        cardsInternal[0] = "new0";
        cardsInternal[1] = "new1";
        cardsInternal[2] = "new2";

        betResultInternal = 6;
        position1Internal = 6;
    }
}

Answer 1

Arrays are reference types which in short means the value of the array is not directly contained within a variable. Instead the variable refers to the value. Hopefully the following code will explain this a bit better (List<T> is also a reference type).

List<int> first = new List<int>()( new int[] {1,2,3});
List<int> second = first;
first.Clear();
Console.WriteLine(second.Count);  // Prints 0

In this scenario there is a List<int> created on the first line which is referred to by variable first. The second line does not create a new list but instead creates a second variable named second which refers to the same List<int> object as first. This logic applies to all reference types.

When you pass the variable cards into the method you do not pass a copy of the full array but instead a copy of the variable cards. This copy refers to the same array object as the original cards. Hence any modifications you make to the array are visible through the original reference.

Answer 2

A variable of a reference type does not contain its data directly; it contains a reference to its data. When you pass a reference-type parameter by value, it is possible to change the data pointed to by the reference, such as the value of a class member. However, you cannot change the value of the reference itself; that is, you cannot use the same reference to allocate memory for a new class and have it persist outside the block. To do that, pass the parameter using the ref or out keyword.

http://msdn.microsoft.com/en-us/library/s6938f28(VS.80).aspx