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<script language="VB" runat="server">
sub Page_Load(obj as Object, e as EventArgs)
if not Page.IsPostBack then
'create an array of colors
dim arrColors() as string = _
{"red", "orange", "yellow", "green", _
"blue", "indigo", "violet"}
lbColors.DataSource = arrColors
end if
DataBind()
end sub
</script>
<html><body>
<form runat="server">
<asp:Listbox runat="server" id="lbColors"
width="150"
AutoPostBack="true"
SelectionMode="Single" >
</asp:Listbox><p>
<asp:Label id="lblMessage" runat="server"
Text='<%# lbColors.SelectedItem.Text %>' />
</form>
</body></html>
*********************************************
错误信息
“/”应用程序中的服务器错误。
-----------------------------
未将对象引用设置到对象的实例。
说明: 执行当前 Web 请求期间，出现未处理的异常。请检查堆栈跟踪信息，以了解有关该错误以及代码中导致错误的出处的详细信息。
异常详细信息: System.NullReferenceException: 未将对象引用设置到对象的实例。
sub Page_Load(obj as Object, e as EventArgs)
if not Page.IsPostBack then
'create an array of colors
dim arrColors() as string = _
{"red", "orange", "yellow", "green", _
"blue", "indigo", "violet"}
lbColors.DataSource = arrColors
end if
DataBind()
end sub
</script>
<html><body>
<form runat="server">
<asp:Listbox runat="server" id="lbColors"
width="150"
AutoPostBack="true"
SelectionMode="Single" >
</asp:Listbox><p>
<asp:Label id="lblMessage" runat="server"
Text='<%# lbColors.SelectedItem.Text %>' />
</form>
</body></html>
*********************************************
错误信息
“/”应用程序中的服务器错误。
-----------------------------
未将对象引用设置到对象的实例。
说明: 执行当前 Web 请求期间，出现未处理的异常。请检查堆栈跟踪信息，以了解有关该错误以及代码中导致错误的出处的详细信息。
异常详细信息: System.NullReferenceException: 未将对象引用设置到对象的实例。
...全文
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yunhi 2002-06-08
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你的code <asp:Label id="lblMessage" runat="server"
Text='<%# lbColors.SelectedItem.Text %>' />
是要绑定控件lbColors的SelectedItem.Text 属性值，但是你在pag_loda中并没有对 lbColors进行数据邦定，因为DataBind()相当于Page.DataBind();
所以lbColors的items为null，那控件lblMessage邦定肯定为null，所以会引发异常。
Text='<%# lbColors.SelectedItem.Text %>' />
是要绑定控件lbColors的SelectedItem.Text 属性值，但是你在pag_loda中并没有对 lbColors进行数据邦定，因为DataBind()相当于Page.DataBind();
所以lbColors的items为null，那控件lblMessage邦定肯定为null，所以会引发异常。
cutesun 2002-06-08
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谢谢 yunhi(yunhi) 根据你的建议，我修改了代码如下
终于可以正常运行了 你知道为什么 开始的代码不能运行么？
我觉得没有错呀！
******************************************************
<script language="VB" runat="server">
sub Page_Load(obj as Object, e as EventArgs)
if not Page.IsPostBack then
'create an array of colors
dim arrColors() as string = _
{"red", "orange", "yellow", "green", _
"blue", "indigo", "violet"}
lbColors.DataSource = arrColors
end if
lbColors.DataBind()
end sub
sub lbColors_SelectedIndexChanged(obj as Object, e as EventArgs)
lblMessage.Text=lbColors.SelectedItem.Text
end sub
</script>
<html><body>
<form runat="server">
<asp:Listbox runat="server" id="lbColors"
width="150"
AutoPostBack="true"
OnSelectedIndexChanged="lbColors_SelectedIndexChanged"
SelectionMode="Single" >
</asp:Listbox><p>
<asp:Label id="lblMessage" runat="server" />
</form>
</body></html>
终于可以正常运行了 你知道为什么 开始的代码不能运行么？
我觉得没有错呀！
******************************************************
<script language="VB" runat="server">
sub Page_Load(obj as Object, e as EventArgs)
if not Page.IsPostBack then
'create an array of colors
dim arrColors() as string = _
{"red", "orange", "yellow", "green", _
"blue", "indigo", "violet"}
lbColors.DataSource = arrColors
end if
lbColors.DataBind()
end sub
sub lbColors_SelectedIndexChanged(obj as Object, e as EventArgs)
lblMessage.Text=lbColors.SelectedItem.Text
end sub
</script>
<html><body>
<form runat="server">
<asp:Listbox runat="server" id="lbColors"
width="150"
AutoPostBack="true"
OnSelectedIndexChanged="lbColors_SelectedIndexChanged"
SelectionMode="Single" >
</asp:Listbox><p>
<asp:Label id="lblMessage" runat="server" />
</form>
</body></html>
yunhi 2002-06-08
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This is C# Code-Binder:
private void Page_Load(object sender, System.EventArgs e)
{
if(!IsPostBack)
{
string[] arrColors={"red", "orange", "yellow", "green", "blue", "indigo", "violet"};
lbColors.DataSource = arrColors;
lbColors.DataBind();
}
}
private void lbColors_SelectedIndexChanged(object sender, System.EventArgs e)
{
lblMessage.Text=lbColors.SelectedItem.Text;
}
aspx Page Code:
<html><body>
<form runat="server">
<asp:Listbox runat="server" id="lbColors"
width="150"
AutoPostBack="true"
SelectionMode="Single" >
</asp:Listbox>
<p>
<asp:Label id="lblMessage" runat="server"/>
</form>
</body></html>
private void Page_Load(object sender, System.EventArgs e)
{
if(!IsPostBack)
{
string[] arrColors={"red", "orange", "yellow", "green", "blue", "indigo", "violet"};
lbColors.DataSource = arrColors;
lbColors.DataBind();
}
}
private void lbColors_SelectedIndexChanged(object sender, System.EventArgs e)
{
lblMessage.Text=lbColors.SelectedItem.Text;
}
aspx Page Code:
<html><body>
<form runat="server">
<asp:Listbox runat="server" id="lbColors"
width="150"
AutoPostBack="true"
SelectionMode="Single" >
</asp:Listbox>
<p>
<asp:Label id="lblMessage" runat="server"/>
</form>
</body></html>
yunhi 2002-06-08
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try again：
sub Page_Load(obj as Object, e as EventArgs)
if not Page.IsPostBack then
'create an array of colors
dim arrColors() as string = _
{"red", "orange", "yellow", "green", _
"blue", "indigo", "violet"}
lbColors.DataSource = arrColors
lbColors.DataBind()
end if
sub Page_Load(obj as Object, e as EventArgs)
if not Page.IsPostBack then
'create an array of colors
dim arrColors() as string = _
{"red", "orange", "yellow", "green", _
"blue", "indigo", "violet"}
lbColors.DataSource = arrColors
lbColors.DataBind()
end if
TINA 2002-06-08
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很可能是在System.Web.UI.Page中
忘了加
protected System.Web.UI.WebControls.Listbox lbColors;
声明
忘了加
protected System.Web.UI.WebControls.Listbox lbColors;
声明
