Today, I write about how Displaying Validation Errors with ASP.NET MVC. We all know how crucial it is to validate user input, but it’s equally crucial how we present validation errors for the user. Take the following type:


The e mail field needs to be optional but any address entered must be valid. If a validation error happens, it ought to replace the optional label using the error message.

Behind the scenes I’m employing Data Annotations to validate the email address, so any errors is going to be passed into the ModelState. MVC has built-in help for displaying these errors (using the ValidateMessageFor helper) so it is easy to create a wrapper around that, supplying the ‘optional’ text as a parameter.

Here’s an extension method for the HtmlHelper:

public static class ExtensionMethods
{
    public static MvcHtmlString HelpMessageFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, string helpText)
    {
        var validation = htmlHelper.ValidationMessageFor(expression);
        if (validation == null)
            return new MvcHtmlString("<span class='help-inline muted'>" + helpText + "</span>");
        else
            return validation;
    }
}

Here’s it’s usage:

<fieldset>
  <div class="control-group">
        @Html.LabelFor(x => x.EmailAddress, "Email", new { @class = "control-label" })
        <div class="controls">
            @Html.TextBoxFor(x => x.EmailAddress, new { @class = "input-xlarge", placeholder = "Enter an email address" })
            @Html.HelpMessageFor(x => x.EmailAddress, "Optional")
        </div>
    </div>
</fieldset>

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