Validation messages are displayed when page load

Question

I have a problem with validation in ASP.NET MVC 2.0. I use the same Action in Controller to perform user request.
For example:

public ActionResult Index(ReportModel model)
{
    if (!model.IsInitialDisplay && ModelState.IsValid)
    {
        model.Result = service.GetResult(model);                
    }
    return View(model);
}  

In the ReportModel, I define a flag IsInitialDisplay to determine whether the page is initial displayed or not:

public class ReportModel
{
    [Required(ErrorMessage = "*")]
    public string Criteria { get; set; }
    public bool IsInitialDisplay { get; set; }
    public ReportResult Result { get; set; }

    public ReportModel()
    {
        IsInitialDisplay = true;
    }
}  

And in the View, I use the following code:

<% using (Html.BeginForm())
   { %>
<table>
    <tr>
        <th>
            Criteria:
        </th>
        <td>
            <%= Html.TextBox("Criteria", "") %>
            <%= Html.ValidationMessage("Criteria") %>
        </td>
    </tr>
</table>
<br />
<input type="submit" value="Submit" />
<%= Html.Hidden("IsInitialDisplay", false) %>
<% } %>  

As I expect, if users don't input any value for Criteria and click Submit button, the error message for validation will be displayed.
But the validation error message always displayed on initial page load, I don't know how to prevent it?
Does anyone know? Thanks,

[Updated]
I have updated my Action method as below and it's seem to be fine:

public ActionResult Index(ReportModel model)
{
    // Collecting some commons data here...

    if (model.IsInitialDisplay)
    {
        ModelState.Clear();
    }
    else if (ModelState.IsValid)
    {
        model.Result = service.GetResult(model);                
    }
    return View(model);
}

Answer 1

The reason an error message is displayed on initial page load is because your controller action takes ReportModel model as argument. When you first access this action with /Home/Index you are passing no arguments and when the default model binder tries to bind to a ReportModel instance it triggers validation errors.

It is a bad practice to use the same action for both rendering and handling the form submission but if you really want to do it you could try like this:

public ActionResult Index(bool? isInitialDisplay)
{
    if (isInitialDisplay.HasValue && !isInitialDisplay.Value)
    {
        var model = new ReportModel();
        UpdateModel(model);
        if (ModelState.IsValid)
        {
            model.Result = service.GetResult(model);                
        }
        return View(model);
    }

    // Initial request
    return View(new ReportModel());
}

In this case you no longer need the IsInitialDisplay property on your model nor the constructor which sets it to true.

This being said, here's the recommended way:

public ActionResult Index()
{
    var model = new ReportModel();
    return View(model);
}

[HttpPost]
public ActionResult Index(ReportModel model)
{
    if (!ModelState.IsValid)
    {
        return View(model);
    }
    model.Result = service.GetResult(model);                
    return View(model);
}