C# 尝试更新SQL Server数据库中的值

目前，我在更新SQL Server数据库中的值时遇到问题

我正在做一个项目，该项目将学生的姓名、出生日期和性别存储在数据库中，并且可以通过windows窗体进行编辑和更新。到目前为止，浏览数据工作正常，但是当表单上的值发生更改并单击“更新”按钮时，程序会崩溃，并出现错误

未处理NullReferenceException。对象引用未设置为对象的实例

错误，我目前不知道是什么导致了它

目前，我有一个DBConnector类，它打开到数据库和数据集的连接，并将其传递到windows窗体，该窗体包含显示值的文本框和浏览数据的导航按钮。下面是这个类和windows窗体的代码，我希望它能比我解释的更好

DBConnector.cs类

studentdetails.cs Windows窗体

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace StudentRecords
{
    public partial class studentdetails : Form
    {
        DataSet StudentDataSet;
        //set variables for limit
        int Limit = 0;
        int current = 0;

        public studentdetails()
        {
            InitializeComponent();
        }

        public studentdetails(DataSet ds)
            : this()
        {
            StudentDataSet = ds;
            //set the limit of records we can navigate
            Limit = StudentDataSet.Tables["Students"].Rows.Count;

            NavigateRecords();
        }

       public studentdetails(DBConnector db) : this()
       { }

        public studentdetails(DBConnector db, int sRow) : this(db)
        {
            current = sRow;
            NavigateRecords();
            //turn on editing
            plEdit.Visible = true;
            //set our local dataset object to point to the passed in one
            DBConnection = db;
            StudentDataSet = db.DBDataSet;
            Limit = StudentDataSet.Tables["Students"].Rows.Count;
            NavigateRecords();
        }

        //navigate records function to move through the records
        public void NavigateRecords()
        {   //create a datarow object and set it to be the first row in the dataset
            DataRow dRow = StudentDataSet.Tables["Students"].Rows[current];
            //set the form text to add the current record number
            this.Text += " for record " + dRow.ItemArray.GetValue(0).ToString();
            //fill the text boxes with the database values
            txtFirstName.Text = dRow.ItemArray.GetValue(1).ToString();
            txtMiddleName.Text = dRow.ItemArray.GetValue(2).ToString();
            txtLastName.Text = dRow.ItemArray.GetValue(3).ToString();
            txtDOB.Text = dRow.ItemArray.GetValue(4).ToString();
            txtgender.Text = dRow.ItemArray.GetValue(5).ToString();
        }

        //update the label for the dtatbase
        private void UpdateCount()
        {
            txtCount.Text = (current + 1).ToString() + " of " + Limit.ToString();
        }

        private void btn_next_Click(object sender, EventArgs e)
        {
            if (current != Limit - 1)
            {
                current++;
                NavigateRecords();
            }
            else
            {
                MessageBox.Show("Last Record", "Information", 0, MessageBoxIcon.Information);
            }
        }

        private void btn_prev_Click(object sender, EventArgs e)
        {
            if (current > 0)
            {
                current--;
                NavigateRecords();
            }
            else
            {
                MessageBox.Show("First Record", "Information", 0, MessageBoxIcon.Information);
            }
        }

        private void btn_Last_Click(object sender, EventArgs e)
        {
            if (current != Limit - 1)
            {
                current = Limit - 1;
                NavigateRecords();
            }
        }

        private void btn_first_Click(object sender, EventArgs e)
        {
            if (current != 0)
            {
                current = 0;
                NavigateRecords();
            }
        }

        public DBConnector DBConnection { get; set; }

        private void btn_save_Click(object sender, EventArgs e)
        {
            {
                //create a new datarow 
                DataRow dRow = StudentDataSet.Tables["Students"].NewRow();

                //set the data to be the values from the text boxes
                dRow[1] = txtFirstName.Text;
                dRow[2] = txtMiddleName.Text;
                dRow[3] = txtLastName.Text;
                dRow[4] = txtDOB.Text;
                dRow[5] = txtgender.Text;


                //add the row to our dataset
                StudentDataSet.Tables["Studentstbl"].Rows.Add(dRow);

                //increase the limit as we have a new record
                Limit++;
                //move to the newly entered Student
                current = Limit - 1;
                DBConnection.UpdateDB(StudentDataSet, "Students");
                MessageBox.Show("Record Saved", "Information", 0, MessageBoxIcon.Information);
                NavigateRecords();
            }



        }

        private void btn_update_Click(object sender, EventArgs e)
        {
            {   //get the current row
                DataRow dRow = StudentDataSet.Tables["Students"].Rows[current];
                //set the dataset values to those in the textboxes
                dRow[1] = txtFirstName.Text;
                dRow[2] = txtMiddleName.Text;
                dRow[3] = txtLastName.Text;
                dRow[4] = txtDOB.Text;
                dRow[5] = txtgender.Text;

                //call the update method in our DB connector class
                DBConnection.UpdateDB(StudentDataSet, "Students");
                //display a message box
                MessageBox.Show("Record updated", "Information", 0, MessageBoxIcon.Information);

                NavigateRecords();
            }

        }






    }
}

数据库名为Students.mdf，表名为Studentstbl，第行抛出错误：

DBConnection.UpdateDB(StudentDataSet, "Students");

我知道这可能是我遗漏了一些愚蠢的东西，但我对C代码还相当陌生，所以如果是这样的话，请原谅我：

编辑：我还应该指出，studentdetails是一个次要表单，主表单只包含一个按钮，可以在其中打开该表单，它包含以下代码：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Coursework3
{
    public partial class Form1 : Form
    {
        DBConnector Students;
        public Form1()
        {
            InitializeComponent();
            //intansiate the object in memory
            Students = new DBConnector();
        }

        private void btnstudentdetails_Click(object sender, EventArgs e)
        {
            new studentdetails(Students.DBDataSet).ShowDialog();
        }
    }
}

---

如果您是新手，请学习使用调试器。。。我敢打赌，如果您能够跟踪特定代码行的异常，您将自己发现问题；同意，在调试模式下运行此命令，您应该正好在导致null引用异常的代码行结束。这是一个未使用值初始化的对象，但正在尝试以某种方式访问它-调用函数、属性等。如果仍有问题，请在post中留下注释，以防止通过调试跟踪它。您的DBConnection为null，几乎可以肯定，因为它只在三个构造函数中的一个中设置。我猜这不是您要调用的。NullReferenceException是.NET堆栈中最常见和最容易修复的异常之一。我的问题是，DBConnection.UpdateDBStudentDataSet，学生；如果在此行抛出异常，那么哪个对象为null？这里的断点可能会告诉您。。。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Coursework3
{
    public partial class Form1 : Form
    {
        DBConnector Students;
        public Form1()
        {
            InitializeComponent();
            //intansiate the object in memory
            Students = new DBConnector();
        }

        private void btnstudentdetails_Click(object sender, EventArgs e)
        {
            new studentdetails(Students.DBDataSet).ShowDialog();
        }
    }
}