当ASP.NET DropDownList.SelectedValue位于FormView编辑项模板中时,如何设置它?

我正在尝试设置DropDownList的选定值,它位于FormView的编辑模板内。每当我访问它时,就会出现错误:

对象引用不存在

我试着用以下方法设置它:

DropDownList ddl = (DropDownList)FormView1.FindControl("ddlFrequency");
ddl.SelectedValue = "blah blah";

而且还喜欢:

((DropDownList)FormView1.FindControl("ddlFrequency")).SelectedValue = "blah blah";

如何设置DropDownList.SelectedValue?

编辑:以下是整个方法:

  protected void btnEdit_Click(object sender, EventArgs e)
{
    String frequency = ((Label)(FormView1.FindControl("lblFrequency"))).Text;

    FormView1.ChangeMode(FormViewMode.Edit);

    String selectedValue = "0";

    switch (frequency.ToLower())
    {
        case "none": selectedValue = "0"; break;
        case "daily": selectedValue = "1"; break;
        case "weekly": selectedValue = "7"; break;
        case "monthly": selectedValue = "28"; break;
        case "bi-monthly": selectedValue = "56"; break;
        case "quarterly": selectedValue = "84"; break;
        case "semi-annually": selectedValue = "168"; break;
        case "annually": selectedValue = "365"; break;
        default: break;
    }

    DropDownList ddl = (DropDownList)FormView1.FindControl("ddlFrequency");
    ddl.SelectedValue = selectedValue;
}