一开始先不注册的时候，直接输入账户，密码。不能报错 “不存在账户”！！

既然如此,那注册的意义何在啊,你不判断账户密码,直接进不就可以了

那QQ还不是一样，没注册就会显示不存在 账户

private void button1_Click(object sender, EventArgs e)
        {
            if (radioButton1.Checked == true)
            {
                if (textBox1.Text == "" || textBox2.Text == "")
                {
                    MessageBox.Show("\n\n登录失败\n\n请填写好登录信息！！");
                }
                else if (PublicS.listS.Count == 0)
                {
                    MessageBox.Show("\n\n登录失败\n\n请先注册！！");
                }
                else
                {
                    IEnumerable<Student1> ie = PublicS.listS.Where((Student1 stu) => stu.Num == textBox1.Text);
                    if (ie.Count() > 0)
                    {
                        Student1 stu1 = ie.First();
                        if (textBox2.Text != stu1.Spw1)
                        {
                            MessageBox.Show("\n\n密码错误！");
                            textBox2.Text = "";
                        }
                        else
                        {
                            StudentBox To = new StudentBox();
                            To.Show();
                            this.Hide();
                        }
                    }
                    else
                    {
                        MessageBox.Show("\n\n学生号不存在！");
                        textBox1.Text = "";
                        textBox2.Text = "";
                    }
                }
                //else if (textBox1.Text != PublicS.listS[0].Num)
                //{
                //    MessageBox.Show("\n\n学生号不存在！");
                //    textBox1.Text = "";
                //    textBox2.Text = "";
                //}
                //else if (textBox2.Text != PublicS.listS[0].Spw1)
                //{
                //    MessageBox.Show("\n\n密码错误！");
                //    textBox2.Text = "";
                //}
                //else if (textBox1.Text == PublicS.listS[0].Num && textBox2.Text == PublicS.listS[0].Spw1)
                //{
                //    StudentBox To = new StudentBox();
                //    To.Show();
                //    this.Hide();
                //}
            }
            if (radioButton2.Checked == true)
            {

                if (textBox1.Text == "" || textBox2.Text == "")
                {
                    MessageBox.Show("\n\n登录失败\n\n请填写好登录信息！！");
                }
                else if (PublicT.listT.Count == 0)
                {
                    MessageBox.Show("\n\n登录失败\n\n请先注册！！");
                }
                else
                {
                    IEnumerable<Teacher1> ie = PublicT.listT.Where((Teacher1 teach) => teach.Num == textBox1.Text);
                    if (ie.Count() > 0)
                    {
                        Teacher1 tech1 = ie.First();
                        if (textBox2.Text != tech1.Spw1)
                        {
                            MessageBox.Show("\n\n密码错误！");
                            textBox2.Text = "";
                        }
                        else
                        {
                            TeacherBox To = new TeacherBox();
                            To.Show();
                            this.Hide();
                        }
                    }
                    else
                    {
                        MessageBox.Show("\n\n教工号不存在！");
                        textBox1.Text = "";
                        textBox2.Text = "";
                    }
                }
                //else if ( textBox1.Text != PublicT.listT[0].Num)
                //{
                //    MessageBox.Show("\n\n教工号不存在！");
                //    textBox1.Text = "";
                //    textBox2.Text = "";
                //}
                //else if (textBox2.Text != PublicT.listT[0].Spw1)
                //{
                //    MessageBox.Show("\n\n密码错误！");
                //    textBox2.Text = "";
                //}
                //else if (textBox1.Text == PublicT.listT[0].Num && textBox2.Text == PublicT.listT[0].Spw1)
                //{
                //    TeacherBox To = new TeacherBox();
                //    To.Show();
                //    this.Hide();
                //}
            }
        }

非常感谢