class Test
{
    private string[] array = new string[] { "Element 1", "Element 2", "Element 3" };
    public string[] Array 
    { 
        get => array; 
        set
        {
            array = value;
            Console.WriteLine("Set Array...");        
        }
    }
}
class Program
{
    static void Main(string[] args)
    {
        Test test = new Test();
        test.Array[1] = "Ersetze dieses Element";
    }
}

class Test
{
    private string[] _array = new string[] { "Element 1", "Element 2", "Element 3" };
    public string this[int index] 
    {
        get => _array[index];
        set 
        {
            onItemChanged(index);
            _array[index] = value;
        }
    }
    
    private void onItemChanged(int index) => Console.WriteLine(index); // todo
}
class Program
{
    static void Main(string[] args)
    {
        Test test = new Test();
        test[1] = "Ersetze dieses Element";
    }
}

class MyList<T> : Collection<T>
{
    protected override void ClearItems()
    {
        base.ClearItems();

        OnChanged();
    }
    protected override void InsertItem(int index, T item)
    {
        base.InsertItem(index, item);

        OnChanged();
    }
    protected override void RemoveItem(int index)
    {
        base.RemoveItem(index);

        OnChanged();
    }
    protected override void SetItem(int index, T item)
    {
        base.SetItem(index, item);

        OnChanged();
    }

    private void OnChanged()
    {
        // ...
    }
}