Question

I’m totally new to C# and I’m having an error that I can’t guess why its happening. This is the context:

I have a DataPair class, which is just that, a pair of data (string, float). I also have another class DataSet which is an array of DataPairs.

DataSet has two private members:

private DataPair [] _datapair;
private int _size;

The constructor of DataSet does nothing with _datapair and set _size to 0.

I fill the DataSet with a method Append, which does something like this:

public void Append(DataPair pair)
{
    _datapair[_size] = new DataPair(pair);
    _size++;
}

I call Append from another method, FillFromFile:

public void FillFromFile(string filepath)
{
    try
    {
        if (System.IO.File.Exists(filepath))
        {
            System.IO.StreamReader sr = new System.IO.StreamReader(filepath);
            string[] currentdata;
            while (sr.Peek() >= 0)
            {
                currentdata = sr.ReadLine().Replace(',', '.').Trim().Split(';');
                this.Append(new DataPair(currentdata[0],  System.Convert.ToSingle(currentdata[1])));
            }

            sr.Close();
        }
    }
    catch (Exception e)
    {
        Console.WriteLine("Error in datafile: {0}", e.ToString());
    }
}

It seems that it should work: It creates (new) a new DataPair for each Append.

But I get this error when executing:

"Object reference not set to an instance of an object" in function Append.

What is happening?

Answer 1

Probably _datapair is not initialized yet. You call _datapair[_size] directly that isn’t initialized yet.
Why not use a Collection btw?

private List<DataPair> _datapair = new List<DataPair>();

public void Append(DataPair pair)
{
    _datapair.Add(pair);
}

_size is then obsolete, you can use _datapair.Count()