Question

I am trying to search a list and then extract some objects of it put them into another list and write the list to a file.

I think that my code is right but when I execute it I get a NullReference exception. If someone could check my code would I would be really grateful.

Here’s my code for the Search method that I use

public void Search( string day)
{
    foreach (Classes c in studentClasses)
    {
        if (c.Day == day)
        {
            studentClassesByDay.Add(c);
        }
    }
}

And that’s the code which adds the new list to a file

private void btnDay_Click(object sender, EventArgs e)
    {
        SelectDayForm selectDay = new SelectDayForm();
        if (selectDay.ShowDialog() == DialogResult.OK)
        {
            l.Search(selectDay.theDay);
            trg.GenerateSortedListReport(l.studentClassesByDay, AppData.byDay);
            MessageBox.Show("A list of sorted student classes written to file " + AppData.byDay+ ".");
           // txtDay.Text =selectDay.theDay;

        }
    }

Answer 1

You haven’t shown on which line the exception occurs (which would be handy) but you also haven’t shown where studentClassesByDay has been instantiated.

That would be the first thing I would check.

The only possibilities in the code you’ve shown are:

• l is null;
• trg is null;
• l.studentClassesByDay is null; or
• AppData is null (though that’s probably not so – it looks like a class).

Simply put some debug statements (or breakpoints in your debugger) before each attempted dereference and check the values.

Based on your comment that the errant line is:

studentClassesByDay.Add(c);

it’s almost guaranteed (as suspected) that you havent actually instantiated this variable. I’d look into that.