Can't keep custom attributes in DOM?

A very simple asp.net webform page.

<asp:DronDownList id="ddl" runat="server">
<asp:Button id="btn" runat="server" Text="Do nothing but post back" />

In Page_Load:

if (!IsPostBack)
{
   ListItem item = new ListItem("text1","value1");
   item.Attributes["custom"] = "CustomValue";
   ddl.Items.Add(item);
}

The html it renders:(which looks good)

<select ...>
  <option value="value1" custom="CustomValue">text1</option>
</select>

After the button is clicked, I view the source, custom="CustomValue" is gone. I know you will say "it's because you put it in a if (!IsPostBack) block". Of course everything will be ok if I remove the if statement. But why other STANDARD attributes are rendered? Since I put it in the if statement, i suppose the output will 开发者_如何学Gobe:

<select ...></select>   // i suppose no options in it! 

Why does ASP.NET "choose" attributes?

Its a trade-off. Control has to persist (non-default)value of every attribute that it supports in the view-state. So there is naturally impact on the view-state size and hence page size. Therefore, it make sense for control developers only to back most commonly used attributes into the view-state. Same has been the case here - where control developers has decided not to back custom attributes into the view-state.

ASP .NET does not save custom attribute in ViewState.