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

To just get the errors from the ModelState, use this Linq:

var modelStateErrors = this.ModelState.Keys.SelectMany(key => this.ModelState[key].Errors);

This will give you one string with all the errors with comma separating

string validationErrors = string.Join(",",
                    ModelState.Values.Where(E => E.Errors.Count > 0)
                    .SelectMany(E => E.Errors)
                    .Select(E => E.ErrorMessage)
                    .ToArray());

<% ViewData.ModelState.IsValid %>

or

<% ViewData.ModelState.Values.Any(x => x.Errors.Count >= 1) %>

and for a specific property...

<% ViewData.ModelState["Property"].Errors %> // Note this returns a collection

Condensed version of @ChrisMcKenzie's answer:

var modelStateErrors = this.ModelState.Values.SelectMany(m => m.Errors);