cd.Command.Text=“Select ans From a1 where num=‘“+textbox.text.Trim()+”’”; dr=cd.ExecuteReader();哪里有错误

这段代码缺少异常处理和数据库连接的打开和关闭操作。应该使用 try-catch-finally 块来处理异常，并在使用完 SqlDataReader 后关闭数据库连接和 DataReader。

以下是一个示例代码，供您参考：

try
{
    string connectionString = "your_connection_string_here";
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        connection.Open();
        string commandText = "SELECT ans FROM a1 WHERE num='" + textbox.Text.Trim() + "'";
        using (SqlCommand command = new SqlCommand(commandText, connection))
        {
            using (SqlDataReader reader = command.ExecuteReader())
            {
                if (reader.Read())
                {
                    string answer = reader.GetString(0);
                    // do something with the answer
                }
                else
                {
                    // handle case where no answer was found
                }
            }
        }
    }
}
catch (Exception ex)
{
    // handle exception here
}
finally
{
    // make sure to close database connection and dispose of objects
}