EPiServer: Example of using IValidate

StartPageValidator.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using EPiServer.Core;
using EPiServer.DataAbstraction;
using EPiServer.DataAnnotations;
using EPiServer.SpecializedProperties;
using EPiServer.Validation;

namespace EPiServerSite5.Models.Pages
{
    public class StartPageValidator : IValidate<StartPage>
    {

        public IEnumerable<ValidationError> Validate(StartPage instance)
        {
            if (!instance.Name.StartsWith("X"))
            {
                return new[]
                {
                    new ValidationError
                    {
                        ErrorMessage = "Should not start with X",
                        PropertyName = "PageName",
                        Severity = ValidationErrorSeverity.Warning
                    }
                };
            }
            return new ValidationError[] { };
        }
    }
}