Get error message if ModelState.IsValid fails?

I have this function in my controller.

[HttpPost]
public ActionResult Edit(EmployeesViewModel viewModel)
{
    Employee employee = GetEmployee(viewModel.EmployeeId);
    TryUpdateModel(employee);

    if (ModelState.IsValid)
    {
        SaveEmployee(employee);
        TempData["message"] = "Employee has been saved.";
        return RedirectToAction("Details", new { id = employee.EmployeeID });
    }

    return View(viewModel); // validation error, so redisplay same view
}

It keeps failing, ModelState.IsValid keeps returning false and redisplaying the view. But I have no idea what the error is.

Is there a way to get the error and redisplay it to the user?

Try this

if (ModelState.IsValid)
{
    //go on as normal
}
else
{
    var errors = ModelState.Select(x => x.Value.Errors)
                           .Where(y=>y.Count>0)
                           .ToList();
}

errors will be a list of all the errors.

If you want to display the errors to the user, all you have to do is return the model to the view and if you haven't removed the Razor @Html.ValidationFor() expressions, it will show up.

if (ModelState.IsValid)
{
    //go on as normal
}
else
{
    return View(model);
}

The view will show any validation errors next to each field and/or in the ValidationSummary if it's present.

If you're looking to generate a single error message string that contains the ModelState error messages you can use SelectMany to flatten the errors into a single list:

if (!ModelState.IsValid)
{
    var message = string.Join(" | ", ModelState.Values
        .SelectMany(v => v.Errors)
        .Select(e => e.ErrorMessage));
    return new HttpStatusCodeResult(HttpStatusCode.BadRequest, message);
}

You can do this in your view without doing anything special in your action by using Html.ValidationSummary() to show all error messages, or Html.ValidationMessageFor() to show a message for a specific property of the model.

If you still need to see the errors from within your action or controller, see the ModelState.Errors property

If Modal State is not Valid & the error cannot be seen on screen because your control is in collapsed accordion, then you can return the HttpStatusCode so that the actual error message is shown if you do F12. Also you can log this error to ELMAH error log. Below is the code

if (!ModelState.IsValid)
{
              var message = string.Join(" | ", ModelState.Values
                                            .SelectMany(v => v.Errors)
                                            .Select(e => e.ErrorMessage));

                //Log This exception to ELMAH:
                Exception exception = new Exception(message.ToString());
                Elmah.ErrorSignal.FromCurrentContext().Raise(exception);

                //Return Status Code:
                return new HttpStatusCodeResult(HttpStatusCode.BadRequest, message);
}

But please note that this code will log all validation errors. So this should be used only when such situation arises where you cannot see the errors on screen.

If anyone is here for WebApi (not MVC) you just return the ModelState object:

return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);

ModelState.Values.SelectMany(v => v.Errors).ToList().ForEach(x => _logger.Error($"{x.ErrorMessage}\n"));

Here's a useful snippet I can copy/paste into my OnPostASync() handler to quickly identify the failing validation item(s):

public async Task<IActionResult> OnPostAsync(string editButton)
    {
        if (!ModelState.IsValid)
        {
            var errors = 
                from value in ModelState.Values
                where value.ValidationState == ModelValidationState.Invalid
                select value;  
            return Page();  // <-- I set a breakpoint here, and examine "errors"
        }
        ...

Unlike some of the other solutions above, I can see both

1. The error message text (ErrorMessage) ... AND ...
2. The specific field that caused the error

Ok Check and Add to Watch:

1. Do a breakpoint at your ModelState line in your Code
2. Add your model state to your Watch
3. Expand ModelState "Values"
4. Expand Values "Results View"

Now you can see a list of all SubKey with its validation state at end of value.

So search for the Invalid value.

It is sample extension

public class GetModelErrors
{
    //Usage return Json to View :
    //return Json(new { state = false, message = new GetModelErrors(ModelState).MessagesWithKeys() });
    public class KeyMessages
    {
        public string Key { get; set; }
        public string Message { get; set; }
    }
    private readonly ModelStateDictionary _entry;
    public GetModelErrors(ModelStateDictionary entry)
    {
        _entry = entry;
    }

    public int Count()
    {
        return _entry.ErrorCount;
    }
    public string Exceptions(string sp = "\n")
    {
        return string.Join(sp, _entry.Values
            .SelectMany(v => v.Errors)
            .Select(e => e.Exception));
    }
    public string Messages(string sp = "\n")
    {
        string msg = string.Empty;
        foreach (var item in _entry)
        {
            if (item.Value.ValidationState == ModelValidationState.Invalid)
            {
                msg += string.Join(sp, string.Join(",", item.Value.Errors.Select(i => i.ErrorMessage)));
            }
        }
        return msg;
    }

    public List<KeyMessages> MessagesWithKeys(string sp = "<p> ● ")
    {
        List<KeyMessages> list = new List<KeyMessages>();
        foreach (var item in _entry)
        {
            if (item.Value.ValidationState == ModelValidationState.Invalid)
            {
                list.Add(new KeyMessages
                {
                    Key = item.Key,
                    Message = string.Join(null, item.Value.Errors.Select(i => sp + i.ErrorMessage))
                });
            }
        }
        return list;
    }
}

i use this method as a generic way to get errors

 public static List<string> GetModelErrros(ModelStateDictionary modelState)
        {
            var errors = modelState.Values.Where(E => E.Errors.Count > 0)
                         .SelectMany(E => E.Errors)
                         .Select(E => E.ErrorMessage)
                         .ToList();

            return errors;
        }

and you may use it

logger.LogInformation("ID {0} input Error  ", string.Join(",", SiteUtils.GetModelErrros(ModelState)));

I have no idea if this is your problem, but if you add a user and then change the name of your application, that user will remain in the database (of course), but will be invalid (which is correct behavior). However, there will be no error added for this type of failure. The error list is empty, but ModelState.IsValid will return false for the login.

publicIHttpActionResultPost(Productproduct) {  
    if (ModelState.IsValid) {  
        //Dosomethingwiththeproduct(notshown).  
        returnOk();  
    } else {  
        returnBadRequest();  
    }  
}

OR

public HttpResponseMessage Post(Product product)
        {
            if (ModelState.IsValid)
            {
                // Do something with the product (not shown).

                return new HttpResponseMessage(HttpStatusCode.OK);
            }
            else
            {
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);
            }
        }

Try

ModelState.Values.First().Errors[0].ErrorMessage