Question

好的，所以我正在尝试完成这项任务我得到了一些课程，首先我想制作它们并制作它们的打印功能。后来我需要为组类创建“addStudent”函数，所以我希望我的数组比需要的大。

所以这是我的代码

static void Main(string[] args)
    {
        Student test = new Student("as", "dsa");
        Student test1 = new Student("Stola", "Stolova");
        Student test2 = new Student("alo", "maloo");
        Student[] students = new Student[5];
        students[0] = test;
        students[1] = test1;
        students[2] = test2;           
        Group grp = new Group("aklas", students);
        grp.print();
        Console.Read();
    }
//^
//this is my main function

class Student
{
    private string firstName;
    private string lastName;
    public string FirstName
    {
        get
        {
            return firstName;
        }
    }
    public Student()
    {
        firstName = null;
        lastName = null;
    }
    public Student(string nameOne, string nameTwo)
    {
        firstName = nameOne;
        lastName = nameTwo;
    }
    public void formatPrint()
    {
        Console.WriteLine("{0,10} {1,10}", firstName, lastName);
    }

// ^这是其中一个类

class Group
{
    private string name;
    private Student[] students;
    public Group()
    {
        name = null;
        students = null;
    }
    public Group(string grpName, Student[] grpStudents)
    {
        name = grpName;
        students = grpStudents;
    }
    public void print()
    {
        Console.WriteLine("{0, 15}", name);
        for(int i = 0; i < students.Length; i ++)
        {
            if (students[i].FirstName != null)
                students[i].formatPrint();
        }
    }
}

// ^这是另一个。

所以问题是我为5个元素声明了数组，里面只有3个元素。一切正常，如果我声明数组有3个元素，但如果他们是4或更多我即时获得该空格式，即使我做了if语句。那我的错误在哪里？谢谢！

Answer 1

您正在调用FirstName对象的属性null。这是NullReferenceException的原因。

您应该检查对象是否为空：

if(students[i] != null)
   students[i].formatPrint();

另请查看此问题：What is a NullReferenceException and how do I fix it?

Answer 2

不要使用use array use genereic List.This会动态增加你的类而不用担心数组的大小。

List<student> cl=new List<student>();
class.add(newstudent);
//to print all  students in class use
foreach(Student r in cl)
{
// print info of students
}

当到达数组

2 个答案: