C# passing a reference type by ref

You may ask yourself why you would ever want to pass a reference type into a method using the ref keyword, or why the C# compiler even allows this. Using ref on a reference type is actually slightly different to not using it. The difference is that the ref keyword makes it a reference (pointer) to the variable, not just the object. This allows assigning to the source variable of the parameter from within the method.

I made a little program to illustrate this:

class Program
{
    static void Main(string[] args)
    {
        List<int> listA = new List<int> { 1, 2, 3 };
        List<int> listB = new List<int> { 1, 2, 3 };

        Update(listA);
        UpdateRef(ref listB);

        Console.WriteLine("listA");
        foreach (var val in listA)
            Console.WriteLine(val);

        Console.WriteLine("listB");
        foreach (var val in listB)
            Console.WriteLine(val);
    }

    static void Update(List<int> list)
    {
        list = new List<int>() { 4, 5, 6 };
    }

    static void UpdateRef(ref List<int> list)
    {
        list = new List<int>() { 4, 5, 6 };
    }
}

Here is the output produced by the program:

listA
1
2
3
listB
4
5
6

Notice how listB contains the new List but listA doesn't. This is because we had a reference to the variable listB.