Viewstate of ascx lost between postbacks

问题:

In my ASP.NET application, I am loading an .ascx dynamically using LoadControl, using the following pattern:

var ctrl = LoadControl("/path/to/control.ascx");
((ControlType)ctrl).SomeProperty = someData;
placeholder.Controls.Add(ctrl);

The control that I add saves the SomeProperty property value directly to ViewState, as follows:

public int? SomeProperty
{
    get { return (int?)ViewState["SomeProperty"]; }
    set { ViewState["SomeProperty"] = value; }
}

After that, the ascx control lives a life on its own and all is well until postback occurs. When the page posts back, suddenly the view state is empty! I suspect this happens because I manipulate the ViewState before I add the instantiated ascx to my page. Also, I can prevent the ViewState from getting lost by adding the following line in the Page_Load() method of my ascx control:

SomeProperty = SomeProperty;

I have to do the above for each and every property to ensure that the ViewState is preserved. Now, is there a prettier way of doing this? Manipulating the ViewState after the instantiated .ascx has been added to the page is not an option - I need the contents of the ViewState in the Page_Init() and Page_Load() methods, which are triggered the instant I add the .ascx to my page.

Thanks.

回答1:

Take a look at the ASP.NET Page Life Cycle and Understanding View State. View State gets loaded after Initialization, so you won't be able to access it in Page_Init. You'd be better off using a hidden field.

If you are dead set on using View State, the earliest you can get to it would be by overriding the LoadViewState method (Remember to call base.LoadViewState before trying to access it though).

回答2:

You also need to add the control to the controls collection BEFORE you set the property. ViewState does not get recorded until after it is added to the controls collection.

placeholder.Controls.Add(ctrl); 
((ControlType)ctrl).SomeProperty = someData; 

回答3:

Keep track of the ID of the UserControl before postback then on postback re-create the control and assign the ID back and it should automatically load the ViewState back in.