在论坛上，动态控件好象是永久的话题。大家都知道要动态控件起作用，PostBack时需要重新生成或装载(LoadControl)，而且需要深入了解其状态的变化过程。
有个同事另谋高就，要离开我们工作的地方了。我给她出了一道出门考题，同时也叫其他手下一起参加。这题目是这样的：
下面两页差别很小，就是一句语句的前后次序有所不同，但PostBack后显示效果有所不同，请解释为什么显示效果不同，并且解释正确显示的那页(你知道是哪页，对么？)中Response.Write的输出结果
TestDyn1.aspx:
<html>
<body>
<form id="form1" runat="server">
<asp:Button id="btn" runat="server" Text="Click Me" OnClick="Button_Click" /> <br/>
静态: <asp:DropDownList id="ddlStatic" runat="server">
<asp:ListItem Text="1" Value="1" />
<asp:ListItem Text="2" Value="2" />
<asp:ListItem Text="3" Value="3" />
</asp:DropDownList> <br/>
动态:
</form>
</body>
</html>
<script language="C#" runat="server">
void Page_Load(Object sender, EventArgs e)
{
DropDownList ddlDynamic = new DropDownList();
ddlDynamic.ID = "ddlDynamic";
form1.Controls.Add(ddlDynamic);
if (!IsPostBack)
{
for (int i=1; i <=3; i++)
ddlDynamic.Items.Add(new ListItem(i.ToString(), i.ToString()));
}
if (IsPostBack)
{
Response.Write("[Page_Load]静态:" + ddlStatic.SelectedIndex + "<BR>");
Response.Write("[Page_Load]动态:" + ddlDynamic.SelectedIndex + "<BR>");
}
}
void Button_Click(Object sender, EventArgs e)
{
DropDownList ddlDynamic = (DropDownList)form1.FindControl("ddlDynamic");
Response.Write("[Button_Click]静态:" + ddlStatic.SelectedIndex + "<BR>");
Response.Write("[Button_Click]动态:" + ddlDynamic.SelectedIndex + "<BR>");
}
</script> |
TestDyn2.aspx:
<html>
<body>
<form id="form1" runat="server">
<asp:Button id="btn" runat="server" Text="Click Me" OnClick="Button_Click" /> <br/>
静态: <asp:DropDownList id="ddlStatic" runat="server">
<asp:ListItem Text="1" Value="1" />
<asp:ListItem Text="2" Value="2" />
<asp:ListItem Text="3" Value="3" />
</asp:DropDownList> <br/>
动态:
</form>
</body>
</html>
<script language="C#" runat="server">
void Page_Load(Object sender, EventArgs e)
{
DropDownList ddlDynamic = new DropDownList();
ddlDynamic.ID = "ddlDynamic";
if (!IsPostBack)
{
for (int i=1; i <=3; i++)
ddlDynamic.Items.Add(new ListItem(i.ToString(), i.ToString()));
}
form1.Controls.Add(ddlDynamic); if (IsPostBack) { Response.Write("[Page_Load]静态:" + ddlStatic.SelectedIndex + "<BR>"); Response.Write("[Page_Load]动态:" + ddlDynamic.SelectedIndex + "<BR>"); } }
void Button_Click(Object sender, EventArgs e)
{
DropDownList ddlDynamic = (DropDownList)form1.FindControl("ddlDynamic");
Response.Write("[Button_Click]静态:" + ddlStatic.SelectedIndex + "<BR>");
Response.Write("[Button_Click]动态:" + ddlDynamic.SelectedIndex + "<BR>");
}
</script> |
了解ViewState与控件生命周期的人，对这题自然不在话下。鉴于Lostinet对此有深入研究，本题禁止他参与，
注：本题是从我在CSDN上回答过的一个问题改编而来的