oledb 连接字符串未初始化

Question

    namespace PCMS
    {
     public partial class frmPlayerInterface : Form
    {
        private OleDbConnection con = new OleDbConnection();
        OleDbCommand com = new OleDbCommand();
        private DataTable dt = new DataTable();
    public frmPlayerInterface(string getUser)
    {
        InitializeComponent();
        con.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\Projects\SDP\PCMS\SDP.accdb";
        lblUser.Text = getUser;
    }

    private void btnEnquire_Click(object sender, EventArgs e)
    {
        frmEnquire frmenq = new frmEnquire();
        frmenq.ShowDialog();

    }

    private void btnTopUp1_Click(object sender, EventArgs e)
    {
        frmTopUp frmTU = new frmTopUp();
        frmTU.ShowDialog();
    }

    private void frmPlayerInterface_Load(object sender, EventArgs e)
    {
        con.Open();
        OleDbCommand comm = new OleDbCommand();

        String sql = "select Balance from PlayerAccount where Player_User=@user";
        comm.Parameters.Add(new OleDbParameter("user", lblUser.Text));
        comm.CommandText = sql;

        OleDbDataReader cursor = comm.ExecuteReader();
        while (cursor.Read())
        {

            lblBalance.Text = cursor["Balance"].ToString();
        }
        con.Close();



    }





}

}

嘿，抱歉，大家又问这个了，但我在过去三个小时里一直在尝试这个问题，并挥舞着白旗。仍然出现相同的错误。

我只想让从数据库中选择的余额值显示在标签中。

谢谢 ><

Answer 1

您没有将连接与命令对象相关联：

    con.Open();
    String sql = "select Balance from PlayerAccount where Player_User=@user";
    OleDbCommand comm = new OleDbCommand(sql, con);

请注意，重用连接并不总是最好的设计。连接在 .NET 中汇集在一起，因此重新创建它们通常不是一项昂贵的操作。更好的设计是将 连接字符串 存储为 class 属性然后在需要时创建连接：

private string ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\Projects\SDP\PCMS\SDP.accdb";  
// or better yet - pull form app.config...

当你使用它时：

String sql = "select Balance from PlayerAccount where Player_User=@user";
using(OleDbConnection con = new OleDbConnection(ConnectionString))
{
    con.Open();
    using(OleDbCommand comm = new OleDbCommand(sql, con))
    {
        ... Add parameters, execute query, return results
    }
}

oledb 连接字符串未初始化

oledb connection string not initialized

c#

ms-access

visual-studio-2012

oledb