提示位于错误消息的小于集合大小部分，因为您正在迭代并包括该大小：

  for （i =  0 ; i <  = ListBox3.Items.Count; i ++）

必须是

  （i =  0 ; i <  ListBox3.Items.Count; i ++）

如理查德所述，您的循环可能会跳过项目，您应该从最后一项开始迭代。

[/编辑]

my code is as follows
i want that i select some items from the list box and the selected items are automatically add on to another list box and remove from the first list box .
in asp.net using c#.

What I have tried:

protected void ListBox3_SelectedIndexChanged(object sender, EventArgs e)
{

}
protected void ListBox4_SelectedIndexChanged(object sender, EventArgs e)
{

}
protected void Button6_Click(object sender, EventArgs e)
{
int i;
for (i = 0; i <= ListBox3.Items.Count; i++)
{
if (ListBox3.Items[i].Selected == true)
{
ListBox4.Items.Add(ListBox3.SelectedItem);
ListBox3.Items.Remove(ListBox3.SelectedItem);
}
}









}
}

The hint is in the "less than size of the collection" part of the error message because you are iterating up to and including that size:

for (i = 0; i <= ListBox3.Items.Count; i++)

It must be

for (i = 0; i < ListBox3.Items.Count; i++) 

[EDIT]
As noted by Richard, your loop might skip items and you should iterating down starting at the last item.
[/EDIT]