/// <summary>
/// 从ViewState中获取某属性值，如果为空则返回默认值nullValue
/// </summary>
/// <typeparam name="V">属性类型</typeparam>
/// <param name="propertyName">属性名称</param>
/// <param name="nullValue">默认值</param>
/// <returns>属性值</returns>
protected V GetPropertyValue <V>(string propertyName, V nullValue)
{
return(ViewState.GetViewStateValue <V>(propertyName, nullValue));
}
