Field Validation HTML

The Field Validation HTML is markup that acts as both a placeholder to display any client-side validation messages for a particular Field as well as displaying any server-side validation messages for that Field. The Field Validation HTML can be:

Specified manually
Created by a Field Generator based on the metadata of the model property being displayed and the Field Configuration specified when it's:
- Displayed as part of a Field
- Output directly from the Form

To use a Field Generator to output the HTML for a standalone Field Validation HTML you can use the <field-validation /> tag, e.g.:

<field-validation for="SomeField" />
<field-validation for="SomeField" fluent-config='c => c.ChainFieldConfigurationMethodsHere()' />
<field-validation for="SomeField" add-validation-class="validation" />

To see the different configuration options check out field configuration.

To use a Field Generator to output the HTML for a standalone Field Validation HTML you can use the ValidationMessageFor extension method on the Form, e.g.:

@f.ValidationMessageFor(m => m.SomeField).ChainedFieldConfigurationMethodsHereAreIgnored()

The ValidationMessageFor extension method looks like this:

        /// <summary>
        /// Creates a standalone validation message to be output in a form for a field.
        /// </summary>
        /// <example>
        /// @using (var f = Html.BeginChameleonForm()) {
        ///     @f.ValidationMessageFor(m => m.PositionTitle)
        /// }
        /// </example>
        /// <typeparam name="TModel">The view model type for the current view</typeparam>        
        /// <typeparam name="T">The type of the field being generated</typeparam>
        /// <param name="form">The form the label is being created in</param>
        /// <param name="property">A lamdba expression to identify the field to render the validation message for</param>
        /// <returns>The HTML for the validation message</returns>
        public static IFieldConfiguration ValidationMessageFor<TModel, T>(this IForm<TModel> form, Expression<Func<TModel, T>> property)
        {
            var config = new FieldConfiguration();
            config.SetField(() => form.GetFieldGenerator(property).GetValidationHtml(config));
            return config;
        }

Default HTML

The HTML for the Field Validation HTML is the same as calling:

@Html.ValidationMessageFor(m => m.SomeField, new { @class = %validationClasses% })

<span asp-validation-for="SomeField" class="%validationClasses%"></span>

The default Field Generator ignores all properties on the Field Configuration when generating the Field Validation HTML apart from the ValidationClasses property, which you can set using the AddValidationClass method.