NullReference when Adding to nested list

问题

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 ?

回答1:

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

You can:

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; }
}

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

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

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; }
}

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.

来源：https://stackoverflow.com/questions/39016363/nullreference-when-adding-to-nested-list

标签

nested-lists