尝试添加到词典列表时出现NullReferenceException

Question

执行此代码时,我在这些行上得到NullReferenceException:

List<Dictionary<Slot, string>> slots = new List<Dictionary<Slot, string>>();
                Dictionary<Slot, string> somedict = new Dictionary<Slot, string>();
                somedict.Add(new Slot(), "s");
                this.slots.Add(somedict);

我无法弄清楚发生了什么.我用正确的项创建了一个dict,但是当我尝试将它添加到列表中时,我只得到一个NullReferenceException ....

我一直在寻找MSDN和这个网站大约2个小时,但没有运气.谁能帮我吗？我只是想将一个字典存储到一个列表中.

namespace hashtable
{
    class Slot
    {
        string key;
        string value;

        public Slot()
        {
            this.key = null;
            this.value = null;
        }
    }

    class Bucket
    {
        public int count;
        public int overflow;
        public List<Dictionary<Slot, string>> slots;
        Dictionary<Slot, string> somedict;

        public Bucket()
        {
            this.count = 0;
            this.overflow = -1;
            List<Dictionary<Slot, string>> slots = new List<Dictionary<Slot, string>>();
            Dictionary<Slot, string> somedict = new Dictionary<Slot, string>();
            somedict.Add(new Slot(), "s");
            this.slots.Add(somedict);
            for (int i = 0; i < 3; ++i)
            {
            }
        }
    }
}

Answer 1

您的Bucket构造函数正在创建一个局部变量,slots但您正在尝试添加 somedict到(未初始化的)Bucket成员slots.

更换

List<Dictionary<Slot, string>> slots = new List<Dictionary<Slot, string>>();

同

this.slots = new List<Dictionary<Slot, string>>();

(与之相同)

slots = new List<Dictionary<Slot, string>>();

您将遇到同样的问题somedict.如果你不是故意成为班级成员Bucket,请不要在那里宣布.如果这样做,请不要在Bucket构造函数中将其声明为局部变量.