ExecuteNonQuery：连接属性尚未初始化

Question

I'm receiving an error that says that the connection property has not been initialized (see https://imgur.com/CTHIabz ). This is confusing, as it is the same code that is in other methods that work just fine. Here is the code:

public bool AddBirthday(string full_name, string month, int day, int year)
{
    if (!string.IsNullOrWhiteSpace(full_name) && month.ToString() != "" && day.ToString() != "" && year.ToString() != "")
    {
        // all required fields were filled out
        // determine which month
        values = birthdayMonths.Where(v => v.Value == month).ToList();

        using (OleDbConnection dbConn = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\qbcdb.mdb"))
        {
            dbConn.Open();

            using (cmd = new OleDbCommand("INSERT INTO birthdays (full_name, birthday_month, birthday_day, birthday_year) VALUES (@full_name, @birthday_month, @birthday_day, @birthday_year)"))
            {
                foreach (KeyValuePair<int, string> key in values)
                {
                    if (key.Value != "")
                    {
                        try
                        {
                           cmd.Parameters.AddWithValue("@full_name", full_name);
                           cmd.Parameters.AddWithValue("@birthday_month", key.Value);
                           cmd.Parameters.AddWithValue("@birthday_day", day);
                           cmd.Parameters.AddWithValue("@birthday_year", year);

                           if (cmd.ExecuteNonQuery() > 0)
                           {
                               return true;
                           } 
                        }
                        catch (Exception e)
                        {
                           MessageBox.Show(e.Message);
                        }
                    }
                    else
                    {
                        MessageBox.Show("No values were given.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                    }
               }
            }

            dbConn.Close();
         }
     }

     return false; 
}

I've gone over the code and checked to make sure the database was in the right location and everything else I could think of. I'm confused on why this work would on another method that uses the same code (almost) when it comes to the database query and binds it to a dvg.

Any help would be appreciated.

Thanks!

Oh btw, it inserts selected values from a listboxes, as demonstrated here:

insertBirthday.Click += (senders, argss) =>
{
     if (btd.AddBirthday(name.Text, birthdays.SelectedItem.ToString(), Convert.ToInt32(dayListBox.SelectedItem.ToString()),
         Convert.ToInt32(yearListBox.SelectedItem.ToString())))
     {
         MessageBox.Show("Birthday Added Successfully", "Success", MessageBoxButtons.OK);
      }
};

Answer 1

将此构造函数用于OleDbCommand ： OleDbCommand构造函数（字符串，OleDbConnection）

Answer 2

You haven't assigned the connection to your OleDbCommand. There are two ways you could to this:

cmd.Connection = dbConn;

or in the constructor:

using (cmd = new OleDbCommand(sql, dbConn)) { ... }

It's also good practice not to open your connection until you are going to use it, ie right before cmd.ExecuteNonQuery() . Since you are using a using block, you also don't need to close the connection, that will be taken care of for you, but it's probably not hurting anything.

I recommend separating your database code from your UI code and validation logic (showing MessageBox and checking values) into a separate "repository" class. All of the data necessary to perform the operation should also be passed in as a parameter to your database code so that you don't have any confusing issues where those values aren't being set somewhere else. Calculate the values to insert before calling the method and then pass those as parameters.