c# - 从数据库创建按钮 - 发生 System.NullReferenceException

Question

这段代码昨天有效，但今天当我开始时，我得到了这个错误。 我不知道发生了什么，数据库仍然连接。

代码:

private void LoginFS_Load(object sender, EventArgs e)
{

    SQLFunctions Lgn = new SQLFunctions();
    Lgn.ConnectionToday();

    SqlCommand cmd = new SqlCommand();
    cmd.Connection = SQLFunctions.conn;

    int NumOfButtons = 50;

    for (int i = 1; i <= NumOfButtons; i++)
    {
        Button btn = new Button();
        {
            btn.Tag = i;
            btn.Dock = DockStyle.Fill;
            btn.Margin = new Padding(10, 10, 10, 10);

            cmd.CommandText = "SELECT username FROM Login where id='" + btn.Tag + "'";

            btn.Text = cmd.ExecuteScalar().ToString(); // <= ERROR
            //ERROR: An unhandled exception of type 'System.NullReferenceException' occurred in.  Additional information: Object reference not set to an instance of an object.
            string btn_name = cmd.ExecuteScalar().ToString();
            btn.Name = btn_name.ToString();

            btn.Click += delegate
            {
                pass_txt.Clear();
                username_txt.Text = btn_name;
                username_lbl.Text = btn_name;
                username_lbl.Visible = true;
                pass_txt.ReadOnly = false;
            };

        }
        users_table.Controls.Add(btn);

    }
    SQLFunctions.conn.Close();
}

我该怎么办，或者为什么会出现此错误？ 谢谢

Answer 1

在应用 ToString() 之前检查 cmd.ExecuteScalar() 是否返回任何值

Return Value Type: System.Object The first column of the first row in the result set, or a null reference
https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.executescalar(v=vs.110).aspx