c#中遇到NullReferenceException异常，求大神
student s;
string x = "11111111";
s._number = x;
s._name = "曾艳";
s._money = 200;
s._category = 1;
users.Add(s._number, s);
提示NullReferenceException异常，
private Dictionary<string,student> users;
private struct student{
public int _category;
public string _number;
public string _name;
public int _money;
}
