mysql - 得到一个异常，说参数已经定义

Question

我正在构建一个从数据库中插入或删除人员的程序。当我单击两次删除用户的按钮时，出现异常:参数已定义。这是一些代码。

        con.ConnectionString = ("server=localhost;userid=root;password= '';database= inventory_control")
        cmd.Connection = con
        con.Open()
        cmd.CommandText = "delete from employee where first_name = @fnametodelete "
        cmd.Parameters.Add("@fnametodelete", MySqlDbType.String).Value = stremployeeTodeleteName
        inttodelete = cmd.ExecuteNonQuery()
        con.Close()
        DataGridView2.Refresh()
        clearEmployee()

Answer 1

屏幕截图显示您正在重用 MySqlCommand。因此，您在每次单击按钮时都添加了参数。而是每次都创建一个新命令。

Using con As New MySqlConnection("server=localhost;userid=root;password= '';database= inventory_control")
    Using cmd As New MySqlCommand("delete from employee where first_name = @fnametodelete", con)
        con.Open()
        cmd.Parameters.Add("@fnametodelete", MySqlDbType.String).Value = stremployeeTodeleteName
        inttodelete = cmd.ExecuteNonQuery()
    End Using
End Using