急！代码出错-已打开此命令相关联的DataReader，必须首先将它关闭

运行的时候，错误提示是：用户未处理InvaliOperationException，已打开此命令相关联的DataReader，必须首先将它关闭。
下面是我的代码：
protected void Button1_Click(object sender, EventArgs e)
{


string connstr1 = ConfigurationSettings.AppSettings["connstr"];
SqlConnection myConnection = new SqlConnection(connstr1);
myConnection.Open();

string querydtr1 = "select * from tea1 where 课程号='" + TextBox1.Text + "'";
SqlCommand com = new SqlCommand(querydtr1, myConnection);
SqlDataReader da = com.ExecuteReader();

if (da.Read())
{
Label1.Text = "请重新输入！";


myConnection.Close();
da.Close();

}
else
{
string querystr = "insert into tea1 values('" + TextBox1.Text + "','" + TextBox2.Text + "','" + TextBox3.Text + "'')";

SqlCommand myCommand = new SqlCommand(querystr, myConnection);
myCommand.ExecuteNonQuery(); //已打开此命令相关联的DataReader，必须首先将它关闭
Label1.Text = "增加数据成功！";


}

myConnection.Close();
da.Close();



}