抛出 C# NullReferenceException：我的 StudentList 静态 class 数组为空

Question

我一直在努力弄清楚我的代码发生了什么。我写了一个应用程序，用户可以通过 GUI 应用程序输入学生的分数。第一个表格显示选项，下一个表格是输入学生信息（姓名，编号，分数），最后一个表格是显示学生信息摘要（学生总数，最高分，最低分，姓名最高分的学生，学生名单）。

为了存储所有学生条目，我创建了一个学生class。我创建了一个静态 Student 数组并将其放在我的 ProgramFunctions class 文件中（这都是静态方法）。

当我尝试运行显示学生摘要的表格时，这就是它崩溃的地方 - 如果我查看 Auto 的选项卡，它告诉我 student[a] 的值为空（我做了使用 for 循环遍历数组中的每个学生对象）。我确实跟踪了学生的添加，它确实表明我已经向学生数组添加了新条目。

我的计算方法（highestMark、lowestMark、average）会抛出异常。这是我的代码：

class ProgramFunctions
{
    private static Student[] studentList = new Student[25];
    private static int counter = 0;

    public static void addNewStudent(Student newStudent)
    {
        if (studentList.Count() == counter)
        {
            MessageBox.Show("The Student List is Full", "List is Full");
        }
        else
        {
            studentList[counter] = newStudent;
            counter++;
        }

    }

    public static void displayErrorMessage(String message, String title)
    {
        MessageBox.Show(message, title);
    }

    public static TextBox validateTextBox(int textboxNumber, TextBox thisTextBox)
    {
        if (textboxNumber.Equals(1)) //Student Name textbox
        {
            if (thisTextBox.Text.Length < 0 || thisTextBox.Text.Length > 100)
            {
                displayErrorMessage("The Student Name specified is out of allowed region (greater than 100 or less than 0 characters. Please fix this", "Student Name Error");
            }
            else
            {
                thisTextBox.Text = thisTextBox.Text.Trim();
                return thisTextBox;
            }
        }
        else if (textboxNumber.Equals(2)) //Student number text box (only 10 characters allowed)
        {
            if (thisTextBox.Text.Length < 0 || thisTextBox.Text.Length > 10)
            {
                displayErrorMessage("The student number you specified is greater than 10 characters or less than 0. Please fix this", "Student Number Error");
            }
            else
            {
                thisTextBox.Text = thisTextBox.Text.Trim();
                return thisTextBox;
            }
        }
        else
        {
            if (thisTextBox.Text.Length > 2 || thisTextBox.Text.Length < 0)
            {
                displayErrorMessage("Invalid Length for exam mark", "Invalid Exam Mark");
            }
            else
            {
                thisTextBox.Text = thisTextBox.Text.Trim();
                return thisTextBox;
            }
        }
        return null;
    }

    public static int getMaximumMarkPosition()
    {
        int highestMark = -999;
        int positionFound = -1; 

        for (int a = 0; a < counter; a++)
        {
            if (studentList[a].getExamMark > highestMark)
            {
                highestMark = studentList[a].getExamMark; //This is where the error would occur
                positionFound = a;
            }
        }

        return positionFound;
    }

    public static int getHighestMark(int position)
    {
        return studentList[position].getExamMark;
    }

    public static int getLowestMark(int position)
    {
        return studentList[position].getExamMark;
    }

    public static string getHighestStudentMarkName(int position)
    {
        return studentList[position].getStudentName;
    }

    public static int getLowestMarkPosition()
    {
        int lowestMark = 999;
        int positionFound = -1;
        int studentMark = 0;

        for (int a = 0; a < studentList.Count(); a++)
        {


            studentMark = studentList[a].getExamMark; //This is where the error would occur
            if (studentMark < lowestMark)
            {
                lowestMark = studentMark;
                positionFound = a;
            }
        }

        return positionFound;
    }

    public static double calculateClassAverage()
    {
        double sum = 0;
        double average = 0;

        for (int a = 0; a < studentList.Count(); a++)
        {
            sum = sum + studentList[a].getExamMark;
        }

        average = sum / studentList.Count();

        return average;
    }

    public static int getTotalNumberOfStudents()
    {
        return counter;
    }

    public static RichTextBox getTextBoxData(RichTextBox thisTextBox)
    {
        thisTextBox.Text = "STUDENT MARK DATA: \n\n";

        for (int a = 1; a < studentList.Count(); a++)
        {
            Student currentStudent = returnStudentInformation(a);
            thisTextBox.Text = thisTextBox.Text + "\n" + currentStudent.getStudentName + "\t\t" + currentStudent.getExamMark + "\t" + currentStudent.getStudentNumber;
        }

        return thisTextBox;
    }

    public static Student returnStudentInformation(int index)
    {
        return studentList[index];
    }

}

}

Answer 1

在我看来，如果学生列表未满，则访问计数器或更高位置的任何索引将导致空对象。我建议只循环到计数器：

for (int a = 1; a < counter; a++)
{
    Student currentStudent = returnStudentInformation(a);
    thisTextBox.Text = thisTextBox.Text + "\n" + currentStudent.getStudentName + "\t\t" + currentStudent.getExamMark + "\t" + currentStudent.getStudentNumber;
}

所有这些都引出了一个问题，为什么在 List<T> 可用并且是为动态数据制作的情况下为动态数据使用静态数组。

抛出 C# NullReferenceException：我的 StudentList 静态 class 数组为空

C# NullReferenceException Thrown: My StudentList static class array is null

c#

nullreferenceexception