错误：ExecuteNonQuery：连接属性尚未初始化

Question

Please help me, I'm currently stuck in this error:

ExecuteNonQuery: Connection property has not been initialized

I read many of the related questions but none of them work. I also searched in Google hoping for the answer but sadly I didn't found any. :(

Here is my code.

namespace jollibee4
{
    public partial class AccountRegister : Form
    {
        public AccountRegister()
        {
            InitializeComponent();
        } 

        private void add_Click(object sender, EventArgs e)
        {
            try
            {
                OleDbConnection con = new OleDbConnection();
                con.ConnectionString = @"Provider = Microsoft.ACE.OLEDB.12.0; Data Source = C:\dbms\jollibee.accdb";
                con.Open();
                OleDbCommand cmd = new OleDbCommand();
                cmd.Connection = con;

                cmd = new OleDbCommand("insert into Employee_Details ([username],[password],[Name],[Middle_Name],[Surname],[address],[account_Type],[Mobile_Number],[BirthDate]) values (?,?,?,?,?,?,?,?,?);");
                cmd.Parameters.AddWithValue("@username", username.Text);
                cmd.Parameters.AddWithValue("@password", password.Text);
                cmd.Parameters.AddWithValue("@Name", name.Text);
                cmd.Parameters.AddWithValue("@Middle_Name", middlename.Text);
                cmd.Parameters.AddWithValue("@Surname", surname.Text);
                cmd.Parameters.AddWithValue("@address", address.Text);
                cmd.Parameters.AddWithValue("@account_Type", accountType.Text);
                cmd.Parameters.AddWithValue("@BirthDate", birthdate.Text);
                cmd.Parameters.AddWithValue("@Mobile_Number", mobilenumber.Text);

                cmd.ExecuteNonQuery();
                con.Close();
                MessageBox.Show("done");
            }
            catch (Exception ex)
            {
                 MessageBox.Show(ex.Message);
            }
        }
    }
}

Answer 1

You are creating the command twice, causing the second instance to lose the connection:

    private void add_Click(object sender, EventArgs e)
    {
        try
        {
            OleDbConnection con = new OleDbConnection();
            con.ConnectionString = @"Provider = Microsoft.ACE.OLEDB.12.0; Data Source = C:\dbms\jollibee.accdb";
            con.Open();
            OleDbCommand cmd = new OleDbCommand();
            cmd.Connection = con;

            //THIS LINE IS THE PROBLEM
            cmd = new OleDbCommand("insert into Employee_Details ([username],[password],[Name],[Middle_Name],[Surname],[address],[account_Type],[Mobile_Number],[BirthDate]) values (?,?,?,?,?,?,?,?,?);");
            cmd.Parameters.AddWithValue("@username", username.Text);
            cmd.Parameters.AddWithValue("@password", password.Text);
            cmd.Parameters.AddWithValue("@Name", name.Text);
            cmd.Parameters.AddWithValue("@Middle_Name", middlename.Text);
            cmd.Parameters.AddWithValue("@Surname", surname.Text);
            cmd.Parameters.AddWithValue("@address", address.Text);
            cmd.Parameters.AddWithValue("@account_Type", accountType.Text);
            cmd.Parameters.AddWithValue("@BirthDate", birthdate.Text);
            cmd.Parameters.AddWithValue("@Mobile_Number", mobilenumber.Text);

            cmd.ExecuteNonQuery();
            con.Close();
            MessageBox.Show("done");
        }
        catch (Exception ex)
        {
             MessageBox.Show(ex.Message);
        }
    }

instead it should be:

cmd.CommandText = "insert into Employee_Details ([username],[password],[Name],[Middle_Name],[Surname],[address],[account_Type],[Mobile_Number],[BirthDate]) values (?,?,?,?,?,?,?,?,?);"