C＃无法更改用户控件的{name}属性

Question

When I tried to change the {name} property of the user controls on the form, the below error occurs:

And here's the "selectedVal" property on "ddlPriority" user control

Here's the ddlPriority_Load function

  Scheduler.DAL.TableClass.Priority pri = new Scheduler.DAL.TableClass.Priority();
            combxPriority.DataSource = pri.selectPriorities();
            combxPriority.DisplayMember = "name";
            combxPriority.ValueMember = "PriorityID";
            Scheduler.DAL.TableClass.Settings set = new Scheduler.DAL.TableClass.Settings();
            if (set.thisTable != null)
                combxPriority.SelectedValue = set.thisTable.DefaultPriorityID;

The ddlPriority user control is just a combo box.

How can I solve this problem?

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Edited:

Below is the selectPriorities method:

properties.cs

public static string DBConnection = ConfigurationSettings.AppSettings["DB_Connection"];

Priority.cs

public DataTable selectPriorities()
        {
            SqlConnection con = new SqlConnection(properties.DBConnection);
            con.Open();
            string sqlQuery = "select name, PriorityID from Priority";
            DataTable dt = new DataTable();
            SqlCommand cmd = new SqlCommand(sqlQuery, con);
            SqlDataReader read = cmd.ExecuteNonQuery();
            dt.Load(read);
            read.Close();
            con.Close();
            return dt;
        }

Answer 1

It looks like combxPriority.SelectedValue is null (no line is selected) and that's why combxPriority.SelectedValue.ToString() throws the exception. You have to make sure combxPriority.SelectedValue is not null before accessing it.