如何在自定义.NET Web控件中指定必需属性？

private string _itemId;

[Browsable(true),
Description("Required identifier for the Item.")]
public string ItemId
{
    get { return _itemId; }
    set
    {
        if (string.IsNullOrEmpty(_itemId))
        {
            _itemId = value;
        }
    }
}
private string _itemId;

[Browsable(true),
Desc