系统管理（雏形）

private void button1_Click(object sender, EventArgs e)
{
SqlConnection conn = new SqlConnection("server=.;database=Cml;user=sa;pwd=123");
conn.Open();
SqlCommand cmd = conn.CreateCommand();
cmd.CommandText = "select * from yonghu where username='"+textBox1.Text+"' and userpass='"+textBox2.Text+"'";
SqlDataReader dr =cmd.ExecuteReader();
if (dr.Read())
{
erzhuye f = new erzhuye();
this.Owner = f; //主窗体归副窗体所有,是为了关闭副窗体的时候，主窗体也一块被关闭
this.Visible = false;//将主窗体隐藏掉
f.WindowState = FormWindowState.Maximized;
f.Show();
}
else
{
MessageBox.Show("登陆失败");
}

}

private void 学生管理ToolStripMenuItem_Click(object sender, EventArgs e)
{
Form1 f = new Form1(); //Form1为学生信息
f.MdiParent = this; //将学生信息的父级设为副窗体的
f.Show();
}

private void 教师管理ToolStripMenuItem_Click(object sender, EventArgs e)
{
Teacher f = new Teacher();  //teacher为教师信息
f.MdiParent = this;//将教师信息的父级设为副窗体的
f.Show();
}

private void 课程管理ToolStripMenuItem_Click(object sender, EventArgs e)
{
Course f = new Course(); //Course为课程信息
f.MdiParent = this; //将课程信息的父级设为副窗体的
f.Show();
}

private void 成绩管理ToolStripMenuItem_Click(object sender, EventArgs e)
{
Score f = new Score();//Score为成绩信息
f.MdiParent = this;//将成绩信息的父级设为副窗体的
f.Show();
}