System.Web.HttpException：不能在DropDownList中选择多个项目
在页面加载，索引0被已被选中。那么这个code语句，选择索引1：
During page load, index 0 was already selected. Then this code statement selected index 1:
dropDownList.Items.FindByValue(myValue).Selected = true;
// assume myValue is found at index 1 of dropDownList.Items
在页面加载完成后，页面上写着：System.Web.HttpException：不能在DropDownList中选择多个项目
On completion of page load, the page reads: "System.Web.HttpException: Cannot have multiple items selected in a DropDownList."
为什么我得到的异常？我该如何解决？
Why did I get the exception? And how can I fix it?
我注意到，这两个指数0和指数1例属性选择设置为true（dropDownList.Items [0] .Selected和dropDownList.Items [1 ] .Selected两者都是如此）。然而，dropDownList.SelectedIndex仍然为0，即使指数1最新设定。
I noticed that both index 0 and index 1 had the properties "Selected" set to true (dropDownList.Items[0].Selected and dropDownList.Items[1].Selected both were true). However, dropDownList.SelectedIndex was still 0, even though index 1 was set most recently.
我试图通过事先清除列表选择解决这个。
I tried resolving this by clearing the list selection beforehand.
dropDownList.ClearSelection();
dropDownList.Items.FindByValue(myValue).Selected = true;
但是，这并没有帮助。发生同样的异常。
But that didn't help. Same exception occurred.
什么了帮助下，将选定值的另一种方式：
What did help, was setting the selected value another way:
dropDownList.SelectedIndex = dropDownList.Items.IndexOf(dropDownList.Items.FindByValue(myValue));
现在的选择变化propogates整个列表。
Now the selection change propogates throughout the list.
所以，不要使用dropDownList.Items [X] .Selected =真/假来改变一个DropDownList的选定值。相反，使用dropDownList.SelectedIndex = X;
So, don't use dropDownList.Items[x].Selected = true/false to change the selected value of a DropDownList. Instead, use dropDownList.SelectedIndex = x;