Question

以下是我的应用程序的示例代码，该代码仅从客户表中检索ID，但是当单击customerIDListBox以查看客户信息时，不会检索相关信息。

public partial class Form1 : Form
{
    OleDbConnection dbConn;

    private void ConnectToDatabase()
    {
        //initialise the connection
        dbConn = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;
        User Id=;
        Password=;
        Data Source=DMarx v1.0.mdb");

        //open connection
        dbConn.Open();
    }

   /* private void DisconnectFromDatabase()
    *{
    *   dbConn.Close();
    *}
    */
    private OleDbDataReader ExecuteQuery(string query)
    {
        /*
         * try...catch because ExecuteReader can throw an exception
         * if the query is incorrect, or if the database has not yet been connected
         */
        try
        {
            //create the command object
            OleDbCommand cmd = dbConn.CreateCommand();
            //assign the query to it
            cmd.CommandText = query;
            //execute the command
            return cmd.ExecuteReader();
        }
        catch (OleDbException)
        {
            //if an exception ocurrs, return nothing
            return null;
        }
    }

    public Form1()
    {
        InitializeComponent();

        //connect to the database
        ConnectToDatabase();

        //select all the customer id's from the database
        string query = "SELECT ID FROM Customer;";
        OleDbDataReader dbReader = ExecuteQuery(query);

        //if any rows were returned
        if (dbReader != null && dbReader.HasRows)
        {
            //dbReader.Read() will return true until there are 
            //no more rows to be read
            while (dbReader.Read())
            {
                customerIDListBox.Items.Add(dbReader["ID"]);
            }
        }

        //we are finished with the database for now, disconnect
      //  DisconnectFromDatabase();
    }

    private void customerIDListBox_SelectedIndexChanged(object sender, EventArgs e)
    {
        ConnectToDatabase();

        //construct the query string 
        string queryString = "SELECT Name, Surname, TelNo FROM";
        queryString = queryString + "Customer WHERE ID = " + customerIDListBox.SelectedItem + ";";

        //execute the query
        OleDbDataReader dbReader = ExecuteQuery(queryString);

        //if we have results
        if (dbReader != null && dbReader.HasRows)
        {
            //we are only expecting one row, so it is not neccessary to iterate through the results
            dbReader.Read();
            nameTextBox.Text = dbReader["Name"].ToString();
            surnameTextBox.Text = dbReader["Surname"].ToString();
            telephoneNumberBox.Text = dbReader["TelNo"].ToString();
        }

        //now to get the clients accounts
        queryString = "SELECT Id, AccountType, Balance FROM Account WHERE ClientId = ";
        queryString = queryString + customerIDListBox.SelectedItem + ";";

        //clear the accounts list (so that only the currently selected client's accounts are shown)
        accountsListBox.Items.Clear();

        //execute the query
        if (dbReader != null && dbReader.HasRows)
        {
            while (dbReader.Read())
            {
                //construct a string with the details
                String accountDetails = dbReader["Id"] + " - ";
                accountDetails = accountDetails + dbReader["AccountType"] + " - ";
                accountDetails = accountDetails + dbReader["Balance"];

                //add the string to the accounts list box
                accountsListBox.Items.Add(accountDetails);
            }
        }

      //  DisconnectFromDatabase();

    }

    private void saveChangesButton_Click(object sender, EventArgs e)
    {
        //make sure the use has selected something before clicking the button
        if (customerIDListBox.SelectedItems.Count > 0)
        {
            ConnectToDatabase();

            //create the query
            String queryString = "UPDATE Customer SET Name = '" + nameTextBox.Text + "', Surname = '";
            queryString = queryString + surnameTextBox.Text + "', TelNo= '";
            queryString = queryString + telephoneNumberBox.Text + "'";
            queryString = queryString + " WHERE Id = " + customerIDListBox.SelectedItem + ";";

            ExecuteQuery(queryString);

        //    DisconnectFromDatabase();
        }
        else
        {
            MessageBox.Show("Please select a customer first!");
        }
    }
}

Answer 1

您是否尝试过调试应用程序？是否抛出任何异常（隐藏在try...catch块中的代码）？尝试单步执行代码并查看正在发生的事情。

根据提供的信息，我可以想到两个可能的错误原因：

customerIDListBox_SelectedIndexChanged实际上没有被添加为SelectedIndexChanged的处理程序，因此当您点击列表框时未被调用
ExecuteQuery可能会为您的查询抛出异常 - 在这种情况下，您的代码会悄然无效

无法检索客户表信息

1 个答案: