What is the point of the key parameter in ModelState.AddModelError in ASP.NET MVC?

Question

I've added validation checks in my controller that modify the ModelState if the validation fails.

For example:

private bool ValidateMoney(string raw, string name, decimal min, decimal max) {
    try {
        var dec = Convert.ToDecimal(raw);

        if (dec < min) {
            throw new ArgumentOutOfRangeException(name + " must be >= " + min);
        }
        else if (dec > max) {
            throw new ArgumentOutOfRangeException(name + " must be <= " + max);
        }
    }
    catch (Exception ex) {
        ModelState.AddModelError(name, ex.GetUserMessage());
    }
    return ModelState.IsValid;
}

However, I never know value to pass for the key parameter in ModelState.AddModelError. (In the example, I just set it to my UI display name.)

What is the parameter for and how should I use it?

The key argument is not stupid at all, maybe the documentation isn't as clear as it should be. :) — splattne, Nov 22 '10 at 09:10

score 63 · Accepted Answer · answered Jul 14 '09 at 01:33

63

The Key is used by the ValidationMessage HTML Helper to know the exact error message to display.

Example:

<%=Html.TextBox("Name") %> <br />
<%=Html.ValidationMessage("Name") %>

the ValidationMessage helper will display the message that has the key "Name" in the ModelState dictionary.

answered Jul 14 '09 at 01:33

6

Validation is used in all sort of applications, not only blogs. – Jul 14 '09 at 01:39
3

@Frank respectfully, isn't the feature cool? The Error shows up straight next to the field you want! I learnt this only after google brought me to this question. – gideon Dec 29 '10 at 14:15
6

the key can be an empty string, but must not be null. If the key is an empty string it can be seen using the Html.ValidationSummary(false) html helper – politus Mar 14 '14 at 17:39

OutstandingBill · Answer 2 · 2018-10-25T23:42:53.293

17

The key parameter can be used to associate the validation error with a form field, and thus control where the message appears on-screen. It can be used with both HtmlHelper-type inputs and with simple HTML inputs.

If you've used @Html.TextBoxFor (or similar) and a @Html.ValidationMessageFor, you can get the key's value from the HTML name of the field being validated (use Inspect Element).

If you've just used an HTML <input>, you can add a validation placeholder using @Html.ValidationMessage("AKeyIMadeUp"), and get a message to appear in it like this: ModelState.AddModelError("AKeyIMadeUp", "The value you entered is no good");.

edited Oct 25 '18 at 23:42

answered Feb 07 '15 at 00:18

OutstandingBill

2,614
26
38

That's what I was wondering; if you're using a helper to generate the field(s), you lose control of the key names, as they're generated by the framework – Tom Lint Jul 24 '20 at 15:08
Best answer so far. Got a question tho. Why does the message show up on GET? – SKREFI Dec 14 '20 at 12:24
1

@SKREFI, not sure I follow. I guess the situation you're talking about is where the page is supported by a GET and a POST in the controller, and normally you'd only expect model validation errors to be present after the POST. But if the view model has attributes such as [Required] (writing on my phone, and can't remember the exact syntax) and it's empty, that might cause the situation you're seeing. Not certain, but that's my first guess. – OutstandingBill Dec 16 '20 at 04:31
I see, I think I have done a little mistake in my code, I don't have that problem anymore. I still have troubles with HTML as in, I can't get all the errors and make a switch case on "error", because I have some special classes I have to set for different divs based on the error. Thank you for your reply and explanation! – SKREFI Dec 16 '20 at 10:55

score 7 · Answer 3 · answered Dec 11 '18 at 02:48

Sorry to Necropost. The above answers didn't have this detail that I thought was useful (it was what I was looking for!!)

In order to create 'Model Wide' Validation errors - then you simply add string.Empty as your Key.

e.g.

ModelState.AddModelError(string.Empty, "This is my Model Level Message");

Thanks to : http://www.tutorialsteacher.com/mvc/htmlhelper-validationsummary for the tip.

score 4 · Answer 4 · answered Aug 02 '17 at 07:20

Actually you can set any validation message while your form submission unsuccessful suppose you make a field in model

[Required]
    [DataType(DataType.Password)]
    [Display(Name = "Current password")]
    public string OldPassword { get; set; }

and while your modelState got invalid you can set error message bind with that field like as.

ModelState.AddModelError("OldPassword", "Current Password do not match ");

then your error message will be bind with field in model named "OldPassword"

What is the point of the key parameter in ModelState.AddModelError in ASP.NET MVC?

4 Answers4

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