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<asp:DropDownList ID="drpBlogCatDesktop" DataTextField="CategoryDisplayName" DataValueField="CategoryName" runat="server" AutoPostBack="true" OnSelectedIndexChanged="drpBlogCat_SelectedIndexChanged">
</asp:DropDownList>

void SelectDropdown(DropDownList drpList, string value)
{
  if (string.IsNullOrEmpty(value))
      return;

  if (drpList.Items.FindByValue(value) != null)
  {
      drpList.Items.FindByValue(value).Selected = true;
  }
}