Ｃ＃·未将对象引用设置到对象的实例
private void frmLogin_Load(object sender, System.EventArgs e)
{
//连接对象
string connectionString = @"Data Source=蒙奇＇Ｄ＇路飞\SURETRUENO;Initial Catalog=Student;Integrated Security=SSPI";
SqlConnection sqlCon = new SqlConnection(connectionString);
sqlCon.Open(); //打开数据库连接
}

private void btnLogin_Click(object sender, System.EventArgs e)
{
//检测用户是否输入用户名
if (this.txtName.Text == string.Empty)
{
MessageBox.Show("请输入用户名！");
this.txtName.Focus();
return;
}
//检测用户是否输入密码
else if (this.txtPwd.Text == string.Empty)
{
MessageBox.Show("请输入密码！");
this.txtPwd.Focus();
return;
}
//读取所填用户名的密码
SqlCommand sqlCom = sqlCon.CreateCommand();
string sql = "SELECT UserName, Password FROM User WHERE (UserName = '" +txtName.Text.Trim()+"')";
   
sqlCom.CommandText = sql;
SqlDataReader sqlRd = sqlCom.ExecuteReader();

//判断是否存在该用户
if (!sqlRd.HasRows)
{
MessageBox.Show("用户名不存在！");
return;
}
   
//读取数据库中的内容，并于当前输入比较
while (sqlRd.Read())
{
//判断用户输入与数据库内容是否匹配
if (sqlRd["Password"].ToString().Trim() != txtPwd.Text.Trim())
{
MessageBox.Show("密码不正确！");
txtName.Focus();
return;
}
else
{
this.Hide();
Student Student=new Student();
Student.ShowDialog();
}
}

//关闭数据库连接
sqlRd.Close();
sqlCon.Close();
this.Close();
}

出错的语句是红色字体的那段代码。谢谢帮忙解决学习

------解决方案--------------------
把这段代码

C# code

//连接对象 
string connectionString = @"Data Source=蒙奇＇Ｄ＇路飞\SURETRUENO;Initial Catalog=Student;Integrated Security=SSPI"; 
SqlConnection sqlCon = new SqlConnection(connectionString); 
sqlCon.Open(); //打开数据库连接

------解决方案--------------------
string connectionString = @"Data Source=蒙奇＇Ｄ＇路飞\SURETRUENO;Initial Catalog=Student;Integrated Security=SSPI";  
SqlConnection sqlCon = new SqlConnection(connectionString);  
sqlCon.Open();

这几句不能放在load事件里

直接在类下面声明吧
string connectionString = @"Data Source=蒙奇＇Ｄ＇路飞\SURETRUENO;Initial Catalog=Student;Integrated Security=SSPI";  
SqlConnection sqlCon = new SqlConnection(connectionString);  

然后在要用的地方打开连接sqlCon.Open();