How to deal with List properties

Question

The list doesn't add anything when the method is called, it throws an exception. How should it be done and is it possible to do without writing this:

public List<string> Mark = new List<string>();

I've done like this and this gives an exception

public class Student
{
    protected List<string> _mark;

    public List<string> Mark
    {
        get { return _mark; }
        set { _mark = value; }
    }

    public void Get()
    {
        Mark.Add("Hello");
    }
}
 static void Main(string[] args)
    {
       Student a = new Student();
        a.Get();
    }

score 2 · Accepted Answer · answered Nov 15 '17 at 20:44

2

That's cause you haven't created an instance of the list and so the error. It should be

private List<string> _mark = new List<string>();

Your property should just return it and no setter needed

public List<string> Mark
{
    get { return _mark; }
}

answered Nov 15 '17 at 20:44

Rahul

76,197
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hnefatl · Answer 2 · 2017-11-15T20:47:28.220

1

You don't initialize _mark. Mark.Add("Hello") causes Mark's getter to return _mark, and calling .Add on that null object causes an exception to be thrown.

You should do eg.

protected List<string> _mark = new List<string>();

edited Nov 15 '17 at 20:47

answered Nov 15 '17 at 20:44

hnefatl

5,860
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`new List<>`?? `List<>` is an unbounded generic type and not closed – Rahul Nov 15 '17 at 20:47
@Rahul Whoops, too much Java recently... Corrected, thanks for pointing it out. – hnefatl Nov 15 '17 at 20:47

score 0 · Answer 3 · answered Nov 15 '17 at 20:52

Regarding "is it possible to do without writing this" (and a new use), the answer is no, because creating a new object might not be your intention. Consider bob in the following (contrived) example:

  List<string> jim = new List<string>();
  List<string> bob;
  //code..
  bob = jim;
  //code...

A more realistic scenario might be a List<List<string>> where you retrieve a List<string> into a variable.

How to deal with List properties

3 Answers3