There is already an open DataReader associated with this Command which must be closed first.

這個錯誤訊息常常發生在使用C#的datareader的時候使用同一個connection。

String currentcustomertb = ConfigurationManager.AppSettings["currentcustomertb"];
cmd.CommandText = "SELECT ip,mac FROM " + db +  ".dbo." + currentcustomertb;
SqlDataReader dr = cmd.ExecuteReader();

String replacecurrent = ConfigurationManager.AppSettings["replacecurrent"];

while (dr.Read())
{
 string ip = dr[0].ToString();
 string mac = dr[1].ToString();

 String execReplaceinto = "exec " + db + ".dbo." + replacecurrent + " @cmmac = '" + mac + "' , @cmip = '" + ip + "'";
 SqlCommand cmdexec = new SqlCommand(execReplaceinto, cn);
 cmdexec.ExecuteNonQuery();
 cmdexec.Dispose();
}

因為這個dr與內部的ExecuteNonQuery，所以就會出現這個錯誤訊息。所以有人建議使用不同的connection來避免這個問題。 或是在連線字串加入：MultipleActiveResultSets=True;