ASP.NET MVC How to convert ModelState errors to json

You can put anything you want to inside the select clause:

var errorList = (from item in ModelState
        where item.Value.Errors.Any() 
        select item.Value.Errors[0].ErrorMessage).ToList();

EDIT: You can extract multiple errors into separate list items by adding a from clause, like this:

var errorList = (from item in ModelState.Values
        from error in item.Errors
        select error.ErrorMessage).ToList();

Or:

var errorList = ModelState.Values.SelectMany(m => m.Errors)
                                 .Select(e => e.ErrorMessage)
                                 .ToList();

2^nd EDIT:
You’re looking for a Dictionary<string, string[]>:

var errorList = ModelState.ToDictionary(
    kvp => kvp.Key,
    kvp => kvp.Value.Errors.Select(e => e.ErrorMessage).ToArray()
);