c# - NullReference when Adding to nested list

Question

I have class

public class Gallery
{
    public string method { get; set; }
    public List<List<object>> gidlist { get; set; }
    public int @namespace { get; set; }
}

Button code

private void button1_Click(object sender, EventArgs e)
{
    List<object> data = new List<object>();
    data.Add(618395);
    data.Add("0439fa3666");

    Gallery jak = new Gallery();
    jak.method = "gdata";
    jak.gidlist.Add(data);
    jak.@namespace = 1;

    string json = JsonConvert.SerializeObject(jak);
    textBox2.Text = json;
}

Here I get System.NullReferenceException. How to add item to gidlist ?

Answer 1

You get it because in now place you initialized the list within jak.

You can:

1. Add a default constructor and initialize list there:

   public class Gallery
   {
       public Gallery()
       {
           gidlist = new List<List<object>>();
       }
   
       public string method { get; set; }
       public List<List<object>> gidlist { get; set; }
       public int @namespace { get; set; }
   }
   

2. If in C# 6.0 then you can use the auto-property initializer:

   public List<List<object>> gidlist { get; set; } = new List<List<object>>()
   

3. If in under C# 6.0 and don't want the constructor option for some reason:

   private List<List<object>> _gidlist = new List<List<object>>(); 
   public List<List<object>> gidlist 
   {
       get { return _gidlist; } 
       set { _gidlist = value; }
   }
   

4. You can just initialize it before using (I don't recommend this option)

   Gallery jak = new Gallery();
   jak.method = "gdata";
   jak.gidlist = new List<List<object>>();
   jak.gidlist.Add(data);
   jak.@namespace = 1;
   

If before C# 6.0 best practice will be option 1. If 6.0 or higher then option 2.