错误显示 "The ConnectionString property has not been initialized" 在 c# 中

Question

这段代码是用于组合框的，我想在其中选择一些索引以显示给我的文本框。

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
    try
    {
        conn.Open();
        cmd.Connection = conn;
        string query = "SELECT * FROM GuestInfo WHERE Groomno= '" + comboBox2.Text + "'";
        db.connectDB();
        db.da.SelectCommand = new OleDbCommand(query, db.conn);
        db.executeQryCommand(query, false);
        maxRecord = db.ds.Tables[0].Rows.Count;
        loadRecords(recordCounter);
        cmd.CommandText = query;
        dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            textBox1.Text = dr["Gname"].ToString();
            textBox2.Text = dr["Gcontactno"].ToString();
        }
        conn.Close();
    }
    catch (Exception er)
    {
        MessageBox.Show("Error! " + er.Message);
    }
}

//我的程序正在完全运行，但不在这个部分。(

Answer 1

您是否使用conn对象在应用程序和数据库源之间建立了连接?您可以使用conn对象作为连接对象，但在此之前，您初始化了connection ?简单地使用like

"SqlConnection conn=new SqlConnection("Connection_Source");"

Answer 2

您必须为连接定义连接字符串，这里我建议您执行命令的最佳方法。

using (OleDbConnection conn = new OleDbConnection("yourconnectionString"))
  {
     conn.Open();
     using (OleDbCommand  cmd =new OleDbCommand("your query text",  conn))
       {
         // execute your command
       }
 }

Answer 3

如果只是从comboBox中选择值并显示在textBox中，那么下面的代码将帮助您…

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
 {
    try
    {
        conn.Open();
        OleDbCommand cmd = new OleDbCommand("SELECT Gname,Gcontactno FROM GuestInfo WHERE Groomno= '" + comboBox2.Text + "'", conn);
        OleDbDataReader  dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            textBox1.Text = dr[0].ToString();
            textBox2.Text = dr[1].ToString();
        }
        conn.Close();
    }
    catch (Exception er)
    {
        MessageBox.Show("Error! " + er.Message);
    }
}