Q:

MVC3 Server validation not firing

I have a ViewModel with a StartDate and an EndDate. Obviously, I need to validate StartDate <= EndDate.
I created the StartDateBeforeEndDate validation attribute and decorated the ViewModel class with it:
public class ValidProgramDisplayStartDateAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        ProgramCreateOrEditViewModel vm = value as ProgramCreateOrEditViewModel;
        if (vm == null) return true; //not our problem

        if (!vm.EndDisplay.HasValue) return true; 
            //if you don't set an end date, you can't be invalid

        return vm.StartDisplay <= vm.EndDisplay;
    }
}

...
[ValidProgramDisplayStartDate(ErrorMessage="The program start display date cannot be after the program display end date.")]
public class ProgramCreateOrEditViewModel
{

    [Required(ErrorMessage = "A program title is required.")]
    [StringLength(255, ErrorMessage = "The program title cannot exceed 255 characters.")]
    public virtual string Title { get; set; }

    private DateTime _startDisplayDate = DateTime.Now;

    [Display(Name = "Display Start Date")]
    [Required(ErrorMessage = "A start display date is required")]
    [DataType(DataType.Date)]
    public virtual DateTime StartDisplay { get { return _startDisplayDate; } set { _startDisplayDate = value; } }

    [Display(Name="Display End Date")]
    [DataType(DataType.Date)]
    public virtual DateTime? EndDisplay { get; set; }

...more properties omitted...
}

The Title is correctly validated and its validation message appears in the validation summary as expected, but a breakpoint on IsValid never fires, and no validation error appears if you give an invalid pairing.
The ValidationAttribute works correctly in a unit test that creates a ViewModel object, populates the dates, creates the ValidationAttribute object and calls its IsValid method. It also works correctly if I set a breakpoint on the POST action in the Controller, and use Visual Studio's immediate window to construct the ValidationAttribute and pass it the received ViewModel.
I don't really care about client side validation at this stage; that's filed under "nice to have but who's got the time"? The server-side validation however, is essential.
View:
@model MyProject.Web.Mvc.Controllers.ViewModels.ProgramCreateOrEditViewModel
@using MvcContrib.FluentHtml
@{
    ViewBag.Title = "Index";
}

<h2>Index</h2>
@using (Html.BeginForm())
{

<fieldset class="program"><legend>Program</legend>
    @if (!ViewData.ModelState.IsValid)
    {
        <p class="error">Please correct all errors listed below. The program cannot be saved while errors remain.</p>
        @Html.ValidationSummary(false)
    }
    @Html.HiddenFor(m => m.Id)
    <h3>
        @Html.LabelFor(m => m.Title)
        @Html.TextBoxFor(m => m.Title)</h3>
    <div id="accordion">
        <h3><a href="#">Dates:</a></h3>
        <div class="section">
            Dates related to <em>displaying</em> the program on our site:<br />
            @Html.LabelFor(m => m.StartDisplay)
            @Html.EditorFor(m => m.StartDisplay, new { _class = "date" })
            @Html.LabelFor(m => m.EndDisplay)
            @Html.EditorFor(m => m.EndDisplay, new { _class = "date enddisplay" })
        </div>
     ... more form fields omitted ...
    </div>
   <p>
        <input type="submit" value="Save" /></p>
</fieldset>
}

And finally, the editor template:
@model DateTime?
@Html.TextBox("", Model.HasValue ? Model.Value.ToString("MM/dd/yyyy") : String.Empty, new { @class = "date" })

A:

The issue appears to be that a class ValidationAttribute will not be processed if there are any model errors in the class properties.  It will only be processed after the ValidationAttributes within the class are all satisfied.
For your purposes, if you move [ValidProgramDisplayStartDate(ErrorMessage="The program start display date cannot be after the program display end date.")] from decorating the class to decorating your Id property, then you will get your server side validation, even if there are other model errors.
counsellorben

