Can I add a ModelState.AddModelError from a Model class (rather than the controller)?

Question

I want to display an error to the user in an ASP.MVC 3 input form using ModelState.AddModelError() so that it automatically highlights the right field and puts the error next to the particular field.

In most examples, I see ModelState.AddModelError() and if(ModelState.IsValid) placed right in the Controller. However, I would like to move/centralize that validation logic to the model class. Can I have the model class check for model errors and populate ModelState.AddModelError()?

Current Code:

// Controller
[HttpPost]
public ActionResult Foo(Bar bar)
{  
    // This model check is run here inside the controller.
    if (bar.isOutsideServiceArea())
        ModelState.AddModelError("Address", "Unfortunately, we cannot serve your address.");

    // This is another model check run here inside the controller.
    if (bar.isDuplicate())
        ModelState.AddModelError("OrderNumber", "This appears to be a duplicate order");

    if (ModelState.IsValid)
    {
        bar.Save();
        return RedirectToAction("Index");
    }
    else
        return View(bar)
}

Desired Code:

// Controller
[HttpPost]
public ActionResult Foo(Bar bar)
{  
    // something here to invoke all tests on bar within the model class

    if (ModelState.IsValid)
    {
        bar.Save();
        return RedirectToAction("Index");
    }
    else
        return View(bar)
}


...
// Inside the relevant Model class
if (bar.isOutsideServiceArea())
    ModelState.AddModelError("Address", "Unfortunately, we cannot serve your address.");

if (bar.isDuplicate())
    ModelState.AddModelError("OrderNumber", "This appears to be a duplicate order");

Answer 1

If you are using MVC 3, you should checkout IValidatableObject, it's what you're after.

Scott Gu mentions it in his MVC3 Intro blog posting.

Answer 2

You can do something like this using custom data annotations or using RuleViolations like what they did in the NerdDinner example.