RequestCommandValidator - Manual Validator. Refer to: http://odetocode.com/blogs/scott/archive/2011/06/29/manual-validation-with-data-annotations.aspx

RequestCommandValidator.cs

using Nte.Http.Model.RequestCommands;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web.Http.ModelBinding;

namespace Nte.Http.Infrastructure
{
    public class RequestCommandValidator
    {
        private readonly IRequestCommand _requestCommand;
        private readonly ValidationContext _context;

        public RequestCommandValidator(IRequestCommand requestCommand)
        {
            if (requestCommand == null)
            {
                throw new ArgumentNullException(nameof(requestCommand));
            }

            _requestCommand = requestCommand;
            _context = new ValidationContext(requestCommand, serviceProvider: null, items: null);
        }

        public bool IsValid(out ModelStateDictionary modelStateDictionary)
        {
            var results = new List<ValidationResult>();
            bool isValid = Validator.TryValidateObject(_requestCommand, _context, results, true);
            if (isValid == false)
            {
                var tempModelStateDictionary = new ModelStateDictionary();
                results.ForEach(result =>
                {
                    foreach (string memberName in result.MemberNames)
                    {
                        tempModelStateDictionary.AddModelError(memberName, result.ErrorMessage);
                    }
                });

                modelStateDictionary = tempModelStateDictionary;
            }
            else
            {
                modelStateDictionary = null;
            }

            return isValid;
        }
    }
}