Conditional Validation with Data Annotations in ASP.NET MVC

In the simple blog engine I’m building, I encountered a scenario where I wanted to display different UI elements depending on whether the user was logged in:

When the user is authenticated, their name and email is known, so it’s unnecessary to display them on the comment input form.  While it would be possible to hide just those fields, I decided to make separate partial views and display them in the Post view with the following code:

<%
    if (Model.AllowComments)
    {
        var commentControlName = Request.IsAuthenticated ? AuthenticatedCommentFormControl" : "CommentFormControl";
        Html.RenderPartial(commentControlName, new DotBlog.Models.ViewModels.CommentViewModel(Model.PostId));
    }
%>

This is the view model I used for both partial views:

public class CommentViewModel
{
    ...

    [Required]
    [StringLength(50)]
    [DisplayName("Name")]
    public string CommenterName { get; set; }

    public DateTime Date { get; set; }

    [Required]
    [StringLength(100)]
    public string Email { get; set; }

    [StringLength(100)]
    [DisplayName("Web Site")]
    public string WebSite { get; set; }

    [Required]
    [StringLength(1024)]
    [DisplayName("Comment")]
    public string CommentText { get; set; }

    ...
}

As you can see, I’m using data annotations to enforce required fields and string lengths.  The problem is, even though the name and email aren’t always displayed, they are still required.  I needed a way to validate certain fields conditionally.  Here are the options I came up with:

• Stop using data annotations and find a different solution for validation
• Use two different view models: one for each partial view
• Use actual conditional data annotations, like the library built by Simon Ince.
• When the form is posted, remove the fields from the model state so they’re not validated.

Let’s look at each of these individually:

Stop Using Data Annotations

Data annotations are not the only way to specify validation rules, but they’re easy to implement.  xVal was fine for ASP.MVC 1.0, but is now deprecated.  Custom validation code with ModelState.AddModelError() is flexible, but requires extra work.

Use Two Different View Models

Since I’m using two partial views for the comment form (one for authenticated users and one for guests), I could use a dedicated view model for each.  One would omit the data annotations on name and email and therefore not cause any validation problems.  This was my original implementation, but lead to unnecessary code duplication and other design problems.

Use Conditional Data Annotations

Simon Ince has built a cool library for conditional annotations that allows you to write code like this:

public class ValidationSample
{
    [RequiredIf("PropertyValidationDependsOn", true)]
    public string PropertyToValidate { get; set; }

    public bool PropertyValidationDependsOn { get; set; }
}

Conditionally Remove Fields from the Model State in the Controller

In the end, this is the solution I went with.  The controller executes this code when the form is posted:

if (Request.IsAuthenticated)
{
    ...

    // We don't need to validate user fields if user is logged in.
    ModelState.Remove("CommenterName");
    ModelState.Remove("Email");
}

By removing the fields from the model state, it won’t be invalid when they are missing.  This method is simple and avoids adding additional dependencies.