Question

我有一个带有一些选项的下拉列表。当用户在下拉列表中更改其中一个选项时，我想创建一个注释。问题是我想要说明所做的改变。例如，当货币下拉列表发生变化时，此代码会创建一个注释。因此，如果当前值为“Euro”并且用户将其更改为“Sterling”，我希望该笔记说“货币从欧元变为英镑”等。我知道通过使用selectedItem我可以获得选择的新值，但是如何我在更改之前得到了下拉列表中的值？

if (cust.Currency.ToString() != ddlCustomerCurrency.SelectedItem.Text)
   {
        Customer.Notes.InsertNote(cust.ID, Company.Current.CompanyID, DateTime.Now, "Currency changed from '" + /*Previous value */ + "' to '" + ddlCustomerCurrency.SelectedItem.Text + "'", CurrentUser.UserID, 1);
   }

Answer 1

您可以将下拉列表的初始值和后续值存储在内存中。然后，每次调用OnChange事件时，只需查看＆＃34;当前＆＃34; value并将其与发送到OnChange处理程序的值进行比较。

编辑：这样的事情可能会起到作用：

public class MyClass
{
    private string _currentSelectedCurrency;

    public void DdlCustomerCurrency_OnChange(object sender, EventArgs e)
    {
        if (cust.Currency.ToString() != ddlCustomerCurrency.SelectedItem.Text)
        {
            Customer.Notes.InsertNote(cust.ID, Company.Current.CompanyID, DateTime.Now, "Currency changed from '" + _currentSelectedCurrency +"' to '" + ddlCustomerCurrency.SelectedItem.Text + "'", CurrentUser.UserID, 1);
            _currentSelectedCurrency = ((DropDownList) sender).Text
        }
    }
}

Answer 2

您可以将当前值保存到ddlDropDown的标记属性，因此您始终将最后选择的值设置为“之前的值”。

样品：

void Main()
{
  var currencies = new List<Currency>() {
   new Currency { Code="EUR", Name="Euro"},
   new Currency { Code="USD", Name="US Dollars"},
   new Currency { Code="AUD", Name="Australian Dollars"},
   new Currency { Code="JPY", Name="Japanese Yen"},
  };

  var customer = new Customer { Id=1, Currency="", Notes=new List<string>()};

  Form f = new Form {Text="Sample"};
  ComboBox ddlCustomerCurrency = new ComboBox { Top=10, Left=10, Tag="", 
      DataSource=currencies, DisplayMember="Name"};
  Button b = new Button {Text = "Show Customer Notes", Top=60, Left = 10};    

  f.Controls.Add(ddlCustomerCurrency);
  f.Controls.Add(b);

  ddlCustomerCurrency.SelectedIndexChanged += (sender, args) => {
    var cmb = sender as ComboBox;
    if (cmb != null)
    {
      var currency = cmb.SelectedItem as Currency;
      var oldValue= cmb.Tag;
      if ( currency != null && oldValue != currency.Name )
      {
        customer.Notes.Add( string.Format(
          "\nOld Currency:{0}, New Currency:{1}, Ticks:{2}",
           oldValue, currency.Name, DateTime.Now.Ticks) );
        cmb.Tag = currency.Name;
      }
    }
  };

  b.Click += (sender,args) => {
    if (customer.Notes.Any ())
      MessageBox.Show( customer.Notes.Last () );
  };

  f.Show();
}

class Currency
{
  public string Code { get; set; }
  public string Name { get; set; }
}

class Customer
{
  public int Id { get; set; }
  public string Currency { get; set; }
  public List<string> Notes { get; set; }
}

如何在更改前在下拉列表中选择值？

2 个答案: