Substracting two pandas Series

Using LINQ:

IEnumerable<ModelError> allErrors = ModelState.Values.SelectMany(v => v.Errors);

foreach (ModelState modelState in ViewData.ModelState.Values) {
    foreach (ModelError error in modelState.Errors) {
        DoSomethingWith(error);
    }
}

See also How do I get the collection of Model State Errors in ASP.NET MVC?.