User Agent Causes MVC DisplayFor ArgumentException: Illegal characters in path

Question

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 repr

Answer 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.

Answer 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?