Always check the ModelState

Ever came across weird behavior in your ASP.NET Views? - I did already several times, and this is what I've learned:

Always check the ModelState.

Most of you probably remember to clear the ModelState when you modify _POST_ed values. But I had to find out, that HttpPost isn't the only way to get a messed up ModelState.
I had a simple Action:

[HttpGet]
public ActionResult Edit(string username)
{
    return View(ComponentFactory
        .GetAdapter<IUserListAdapter>()
        .Get(username));
}

and in the view, I just wanted to show a form for editing those details:

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()
    @Html.ValidationSummary(true)
    <fieldset>
        <legend>Globale Benutzer</legend>
        <div class="editor-label">
            @Html.LabelFor(model => model.UserId)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.UserId)
            @Html.ValidationMessageFor(
                model => model.UserId)
        </div>
        <div class="editor-label">
            @Html.LabelFor(model => model.UserName)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.UserName)
            @Html.ValidationMessageFor(
                model => model.UserName)
        </div>
        <div>
            <input type="submit"
                value="@LocalizationHelper
                .LocalizedLiteral("Save").ToString()"
            />
        </div>
    </fieldset>
}

While the value for Model.UserName was the correct name of the user, the text-box (created by @Html.EditorFor(model => model.UserName)) contained the user's id. The same value as Model.UserId. Everything else (label, validation rules, ...) was correct and I couldn't figure out WHY.

What else could modify it?

After no answer from Stack Overflow and some more googling I came across the term ModelState. Since I had no other clue, I thought to give it a try:

[HttpGet]
public ActionResult Edit(string username)
{
    ModelState.Clear();
    return View(ComponentFactory
        .GetAdapter<IUserListAdapter>()
        .Get(username));
}

And .... It Worked!

How does ModelState work?

I didn't give too much attention to ModelState before, just cleared it if it solved my problems and that's all. Yet, I didn't understand, why I would need ModelState.Clear() in a an action as simple as this one.

So I had a deeper look, and it is actually very interesting what happens in the background.
OK, what ASP.NET does is:

1. Taking all parameters you passed to the action (via GET or POST - that doesn't matter)
2. Storing them in a dictionary, the ModelState (which you can empty with ModelState.Clear()).

Later, when the view is created and @Html.EditorFor is called, ASP.NET checks if it has a parameter with that name. (Name checks are case-in-sensitive.) If the parameter is found in the ModelState, it is retrieved from there, instead of evaluating the Model.

So why did I have problems with it?

Now the solution is simple: My Action had the parameter named username. Even I stored the UserId inside. Later when @Html.EditorFor(model => model.UserName) was called, username from the ModelState was retrieved. However this username contained the UserId, passed to the action.

Possible solutions:

• Call ModelState.Clear() (really bad solution, don't like it)
• Rename username to userid, than the ModelState will only replace one userId, for the same from a different source.