添加到嵌套列表时为NullReference [英] NullReference when Adding to nested list

问题描述

我上课

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

按钮代码

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;
}

在这里，我得到System.NullReferenceException.如何将项目添加到清单列表中?

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

推荐答案

之所以得到它，是因为现在您在jak中初始化了列表.

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

您可以:

1. 添加一个默认的构造函数并在那里初始化列表:

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; }
}

如果在C#6.0中，则可以使用自动属性初始化程序:

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

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

如果在C#6.0中，并且不希望使用某些构造函数选项 原因:

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; }
}

您可以在使用前初始化它(我不建议使用此选项)

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;

如果在C#6.0之前，最佳实践将是选项1.如果是6.0或更高版本，则是选项2.

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

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