Theme 'XXX' cannot be found in the application or global theme directories

Question

My asp.net site allows users to pick the theme they want from a list generated from the app_themes folder. From time to time, themes are renamed or removed. Any user who has selected a deleted theme name (it is stored in a cookie) will get the exception:

Theme 'XXX' cannot be found in the application or global theme directories
Stack Trace: 
[HttpException (0x80004005): Theme 'test' cannot be found in the application or global theme directories.]
   System.Web.Compilation.ThemeDirectoryCompiler.GetThemeBuildResultType(String themeName) +920
   System.Web.Compilation.ThemeDirectoryCompiler.GetThemeBuildResultType(HttpContext context, String themeName) +73
   System.Web.UI.Page.InitializeThemes() +8699455
   System.Web.UI.Page.PerformPreInit() +38
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +282

Where is the best place to trap and handle this exception?

Answer 1

You have to make sure that you change users' theme preference if they use your theme to be renamed/deleted. If renamed, then rename accordingly, if deleted, change to default theme.As you store theme preference inside cookies you'll have to check them and make the change on user access.

Answer 2

In the Page_PreInit method where you assign themes, there's a couple of ways to deal with it. What I do is check to make sure that the directory exists. If it does, then that's the theme I want. If it doesn't, then use a default theme where I know the directory exists.

void Page_PreInit(object sender, EventArgs e)
{
    if (ViewState["PageTheme"] == null)
    {
        if (!Directory.Exists("~/App_Themes/THEMENAME_TO_LOOK_FOR"))
        {
            Theme = "DEFAULT_THEME"
        } 
        else 
        {
            Theme = "THEMENAME_TO_LOOK_FOR";
        }
        ViewState["PageTheme"] = Theme;
    } 
    else 
    {
        Theme = ViewState["PageTheme"].ToString();
    }
}

I usually store in the viewstate so I don't have to recheck every time but if you're changing themes on-the-fly, then you'll probably need to not do that.