How do I overload the [] operator in C#

class A
{
    private List<int> values = new List<int>();

    public int GetValue(int index)
    {
        return values[index];
    }
}

public int this[int key]
{
    get
    {
        return GetValue(key);
    }
    set
    {
        SetValue(key,value);
    }
}

class SampleCollection<T>
{
    private T[] arr = new T[100];
    public T this[int i]
    {
        get
        {
            return arr[i];
        }
        set
        {
            arr[i] = value;
        }
    }
}

// This class shows how client code uses the indexer
class Program
{
    static void Main(string[] args)
    {
        SampleCollection<string> stringCollection =
            new SampleCollection<string>();
        stringCollection[0] = "Hello, World";
        System.Console.WriteLine(stringCollection[0]);
    }
}

public int this[int index]
{
  get
  {
     return GetValue(index);
  }
}

public int this[int index]
{
  get
  {
     return GetValue(index);
  }

  set
  {
    SetValue(index, value);
  }
}

public int this[string index]
...

public int this[int index]
    {
        return values[index];
    }