I have a class with a data model and I try to add data items into it.

When I add the items like following it is working:

class DataItem
{
    public int Day { get; set; }
    public double Value { get; set; }
}

 DataItem[] value2023 = new DataItem[] {
    new DataItem
    {
        Day = 1,
        Value = 10
    },
    new DataItem
    {
        Day = 2,
        Value = 20
    },
    new DataItem
    {
        Day = 3,
        Value = 30
    }
  };

When I add the items like following it is not working.Because the data collection is empty I get System.NullReferenceException when I try to reach the elements of the data collection. (Method FillData is called initialy)

class DataItem
{
    public int Day { get; set; }
    public double Value { get; set; }
}
public void FillData()
{
    DataItem[] value2023 = new DataItem[5];
    for (int i=0; i<5;i++)
    {
    value2023.Append(new DataItem { Day = i+1, Value = i*10});
    }
}

What could be the problem? If .Append is the wrong command, how else can I add items programmatically to my data collection?