"未将对象引用设置到对象的实例"
private DateTime dt;
  private void btnOK_Click(object sender, EventArgs e)
  {
  dt = new DateTime(DateTime.Now.Year,DateTime.Now.Month,DateTime.Now.Day,(int)cbbHour.SelectedValue,(int)cbbMinute.SelectedValue,0,0);
  //string ss=txtRemind.Text;
  timer1.Enabled=true;
  }
  private void timer1_Tick(object sender, EventArgs e)
  {
  if (DateTime.Now == dt)
  {
  MessageBox.Show("时间到了！");
  timer1.Enabled=false;
  }
  }

------解决方案--------------------
dt = new DateTime(DateTime.Now.Year,DateTime.Now.Month,DateTime.Now.Day,(int)cbbHour.SelectedValue,(int)cbbMinute.SelectedValue,0,0);

明显红色部分出现了null情况