I found a good write-up of the PRG pattern in MVC by Matt Hawley this week, and have decided to use it in an MVC project I’m working on.  I have made a few changes to Matt’s code however. 

1 - Use ModelBinders and ModelState to Simplify Code

Firstly, as the new version of MVC (the beta release) supports Model Binders, I updated the example to use these instead.  Now we can just save the ModelState into TempData in one go, instead of saving the error messages and the users input, so the Submit action looks something like:

public ActionResult Submit()
{
    //OMITTED: Do work here...
    if (!ModelState.IsValid)
    {
        //Save the current ModelState to TempData:
        TempData["ModelState"] = ModelState;
    }
}

In the Create action we just need to pull these values out of TempData and add to the ModelState.  MVC will then enter the users input back into the textboxes for you.  The Create action looks like:

public ActionResult Create()
{
    //If we have previous model state in TempData, add it to our
    //current ModelState property.
    var previousModelState = TempData["ModelState"] as ModelStateDictionary;
    if (previousModelState != null)
    {
        foreach (KeyValuePair<string, ModelState> kvp in previousModelState)
            if (!ModelState.ContainsKey(kvp.Key))
                ModelState.Add(kvp.Key, kvp.Value);
    }

    return View();
}

(You will want to wrap this boiler plate code up in a helper class or something though)

2. Fix Scenario where user input will be lost

Secondly, I did find a small issue with the code as posted, when the user does the following:

• GET the page with a form
• POST the form with invalid input
• REDIRECT back to the page (with the users input intact)
• REFRESH the page - the users input is now lost!

I’m not sure this is a particularly common scenario, but losing the users input is never a good way to instil trust in your application!  The reason the data is lost in this case is because we stored the users input in TempData which only exists for this request and the next one.  I thought about putting the value into Session instead, but then you’d have to come up with a strategy for removing the items at the right time (you wouldn’t want the form to remember it’s values from the last time it was used for instance).  In the end I decided that just putting the values back into TempData would be the best solution.  This requires the following line to be added to the Create action:

public ActionResult Create() 
{ 
    //If we have previous model state in TempData, add it to our 
    //current ModelState property. 
    var previousModelState = TempData["ModelState"] as ModelStateDictionary; 
    if (previousModelState != null) 
    { 
        foreach (KeyValuePair<string, ModelState> kvp in previousModelState) 
            if (!ModelState.ContainsKey(kvp.Key)) 
                ModelState.Add(kvp.Key, kvp.Value); 
        TempData["ModelState"] = ModelState;  //Add back into TempData
    } 

    return View(); 
}

If the user now refreshes the page after a validation failure, they will no longer lose their input.  If they go on to fix the validation errors and submit the form, the saved TempData value will be automatically cleared by the MVC framework.