User Agent Causes MVC DisplayFor ArgumentException: Illegal characters in path

I’m having a problem where users on mobile devices are encountering an error in MVC that does not occur when viewing the site on a regular desktop. I can consistently reproduce the error by using Chrome’s developer tools and applying any other UA than the default.

The underlying exception thrown is:
ArgumentException: Illegal characters in path.
at System.IO.Path.CheckInvalidPathChars(String path, Boolean checkAdditional)
at System.IO.Path.GetExtension(String path)
at System.Web.WebPages.DefaultDisplayMode.TransformPath(String virtualPath, String suffix)
at System.Web.WebPages.DefaultDisplayMode.GetDisplayInfo(HttpContextBase httpContext, String virtualPath, Func'2 virtualPathExists)
at System.Web.WebPages.DisplayModeProvider.GetDisplayInfoForVirtualPath(String virtualPath, HttpContextBase httpContext, Func'2 virtualPathExists, IDisplayMode currentDisplayMode, Boolean requireConsistentDisplayMode)
at System.Web.Mvc.VirtualPathProviderViewEngine.GetPathFromGeneralName(ControllerContext controllerContext, List'1 locations, String name, String controllerName, String areaName, String cacheKey, String[]& searchedLocations)
at System.Web.Mvc.VirtualPathProviderViewEngine.GetPath(ControllerContext controllerContext, String[] locations, String[] areaLocations, String locationsPropertyName, String name, String controllerName, String cacheKeyPrefix, Boolean useCache, String[]& searchedLocations)
at System.Web.Mvc.VirtualPathProviderViewEngine.FindPartialView(ControllerContext controllerContext, String partialViewName, Boolean useCache)
at System.Web.Mvc.ViewEngineCollection.<>c__DisplayClass2.<FindPartialView>b__1(IViewEngine e)
at System.Web.Mvc.ViewEngineCollection.Find(Func'2 lookup, Boolean trackSearchedPaths)
at System.Web.Mvc.ViewEngineCollection.FindPartialView(ControllerContext controllerContext, String partialViewName)
at System.Web.Mvc.Html.TemplateHelpers.ExecuteTemplate(HtmlHelper html, ViewDataDictionary viewData, String templateName, DataBoundControlMode mode, GetViewNamesDelegate getViewNames, GetDefaultActionsDelegate getDefaultActions)
at System.Web.Mvc.Html.TemplateHelpers.TemplateHelper(HtmlHelper html, ModelMetadata metadata, String htmlFieldName, String templateName, DataBoundControlMode mode, Object additionalViewData, ExecuteTemplateDelegate executeTemplate)
at System.Web.Mvc.Html.TemplateHelpers.TemplateHelper(HtmlHelper html, ModelMetadata metadata, String htmlFieldName, String templateName, DataBoundControlMode mode, Object additionalViewData)
at System.Web.Mvc.Html.TemplateHelpers.TemplateFor[TContainer,TValue](HtmlHelper'1 html, Expression'1 expression, String templateName, String htmlFieldName, DataBoundControlMode mode, Object additionalViewData, TemplateHelperDelegate templateHelper)
at System.Web.Mvc.Html.TemplateHelpers.TemplateFor[TContainer,TValue](HtmlHelper'1 html, Expression'1 expression, String templateName, String htmlFieldName, DataBoundControlMode mode, Object additionalViewData)
at System.Web.Mvc.Html.DisplayExtensions.DisplayFor[TModel,TValue](HtmlHelper'1 html, Expression'1 expression)

Using fiddler, the only difference in requests when comparing a successful one to a failed request is the User-Agent (and the cache buster appended by jQuery as part of the query string parameters).

Why is only changing the UA causing this exception and how can I avoid this issue without writing a specific work around into the system for every place that this is and could occur?

Answers:

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Method 1

I had the exact same problem, and fixed it.

My problem turned out to be the use of a yield block in my viewmodel:

Controller:

var vm = new BigVM {
    SmallVMs = BuildSmallOnes()
};
return View(vm);

private IEnumerable<SmallVM> BuildSmallOnes()
{
    // complex logic
    yield return new SmallVM(1);
    yield return new SmallVM(2);
}

View:

@model BigVM
@Html.DisplayFor(x => x.SmallVMs)   <-- died

Inexplicably, this worked for desktops but failed for iPads and iPhones, citing the exact same stacktrace. Similar problems were reported here and here. The problem was solved by adding a .ToList() call, thus:

var vm = new BigVM {
    SmallVMs = BuildSmallOnes().ToList()
};

Presumably the class that the compiler generates to represent the yield block includes some characters that some User Agents just don’t like. Including the ToList() call uses a List<> instead.

Method 2

I believe that the reason we are having this issue is MultipleViews and DisplayMode providers introduced for handling mobile devices separately. This is code that is still in .NET 4 but has been deprecated.

A solution to disable the functionality entirely can be found here.
Illegal characters in path depending on User-Agent?

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0