c# - 列表框 Selected.value 抛出空异常

Question

填充列表框如下：

if (ds != null)
{
    ListPreviousRecords.Items.Clear();

    ListPreviousRecords.DataSource = ds;
    ListPreviousRecords.DataTextField = "Date";
    ListPreviousRecords.DataValueField = "ID";
    ListPreviousRecords.DataBind();
}

获取选定的值：

protected void ListPreviousRecords_OnSelectedIndexChanged(object sender, EventArgs e)
{
    if(ListPreviousRecords.SelectedItem.Value != "")
    {
        int mySelectedValue = int.Parse(ListPreviousRecords.SelectedItem.Value);// throwing null exception
        loadPreviousDetails(mySelectedValue);
    }
}

Answer 1

您可以添加此代码以确保输入非空值

if(!string.IsNullOrEmpty(ListPreviousRecords.SelectedItem.Value ))
{
...
}

并确保AutoPostBack="true"在您的控制下设置

链接：http: //msdn.microsoft.com/fr-fr/library/system.string.isnullorempty.aspx

Answer 2

改变：

 if(ListPreviousRecords.SelectedItem.Value != "")

到：

  if (!string.IsNullOrEmpty(ListPreviousRecords.SelectedItem))

Answer 3

使用ListPreviousRecords.SelectedValue.

if (!string.IsNullOrWhiteSpace(ListPreviousRecords.SelectedValue)) {
    // ...
}