ASP MVC Validation

Config

In MVC 3+ it's on by default:

Model

Use Data Annotations (Required, StringLength, RegularExpression, Range). NB: [DataType] is just a UI hint, it won't enforce numbers etc.

View Layout

For client side validation, you need jquery.validate.min.js (github for localizations) and jquery.validate.unobtrusive.min.js. They are already installed (but update in Nuget).

You can't put the Html.BeginForm() and @Html.ValidationSummary() in the _Layout.cshtml.
It needs to be in the page view for unobtrusive validation to work.

Views

@using (Html.BeginForm()) {
    @Html.ValidationSummary()
    @Html.EditorForModel(Model)
    <input type="submit" value="Save"/>
}

@Html.ValidationSummary is just like the webforms validation summary.
There's an overload which doesn't show property annotations: @Html.ValidationSummary(excludePropertyErrors:true)
Html.ValidationSummary must be inside the form (and NOT in the _Layout)
@Html.EditorForModel(Model) may be useful for simple models
@Html.EditorFor (or @Html.TextBoxFor etc) render the inputs with data attributes, which the unobtrusive javascript uses
@Html.ValidationMessageFor is just a placeholder for property validations (may duplicate @Html.ValidationSummary)

Controllers

[HttpPost]
public ActionResult Edit(CategoryModel category)
{
    if (!ModelState.IsValid)
    {
return View("Edit", category); //redisplay
    }
    _repository.Save(category); //save it
    return RedirectToAction("Index");
}

You can add model errors with ModelState.AddModelError(key, string) (fully model-level errors have key: "")

ModelState.AddModelError(key:"", errorMessage:"Invalid");

Custom validators

Brad Wilson MVC 3 intro
Data Annotations: write a ValidationAttribute and override IsValid(value, context)
Also implement IClientValidable GetClientValidationRules(modelMetadata, context).
You return (an enumerable of) ModelClientValidationRule, which has an ErrorMessage (use metadata.GetDisplayName()), ValidationParameters (inputs) and ValidationType (js method name)
Javascript adaptor (one of add, addBool, addSingleValue, addMinMax):

$.validator.unobtrusive.adapters.
addBool("validcreditcard");
Add the javascript validator:

$.validator.addMethod("validcreditcard",
    function (value, element) {
        return !isNaN(parseInt(value, 10));
    });

static void