private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
string sql = string.Format("select ClassName from C where ClassNo = '{0}'",classname1.Text);
SqlCommand cmd = new SqlCommand(sql,dbhelp.hong);
dbhelp.hong.Open();
cmd.CommandText = sql;
classname1.Text = cmd.ExecuteScalar().ToString();
dbhelp.hong.Close();
}这段代码会出现未将对象引用到对象实例中，请问这是怎么回事，要怎么改（请附代码），谢谢了
{
string sql = string.Format("select ClassName from C where ClassNo = '{0}'",classname1.Text);
SqlCommand cmd = new SqlCommand(sql,dbhelp.hong);
dbhelp.hong.Open();
cmd.CommandText = sql;
classname1.Text = cmd.ExecuteScalar().ToString();
dbhelp.hong.Close();
}这段代码会出现未将对象引用到对象实例中，请问这是怎么回事，要怎么改（请附代码），谢谢了
2. cmd.ExecuteScalar().ToString(); 数据没取到。
未处理 System.NullReferenceException
Message="未将对象引用设置到对象的实例。"
Source="MyLastWork"
StackTrace:
在 MyLastWork.Classmod.comboBox2_SelectedIndexChanged(Object sender, EventArgs e) 位置 C:\Documents and Settings\Administrator\桌面\MyWork\MyLastWork\MyLastWork\Classmod.cs:行号 199
是cmd.ExecuteScalar().ToString(); 这行错误
private static string conn = "data source =.; initial catalog = Mydatabase3; UID = sa;PWD = 0";
public static SqlConnection hong = new SqlConnection(conn);
string sql = string.Format("select ClassName from C where ClassNo = '{0}'",comboBox1.SelectedText);