MainPageViewModel.cs

using System;
using System.ComponentModel;
using System.Windows.Input;
using EntryValidationBorder.Models;
using Xamarin.Forms;

namespace EntryValidationBorder.ViewModels
{
    public class MainPageViewModel: INotifyPropertyChanged
    {
        public User User { get; set; }
        public bool ErrorMessageVisiliby { get; set; }
        public ICommand OnValidationCommand { get; set; }
        public MainPageViewModel()
        {
            User = new User();
			
			OnValidationCommand = new Command((obj) =>
            {
				User.FirstName.NotValidMessageError = "Name is required";
				User.FirstName.IsNotValid = string.IsNullOrEmpty(User.FirstName.Name);

				User.Email.NotValidMessageError = "Email is required";
				User.Email.IsNotValid = string.IsNullOrEmpty(User.Email.Name);

               
                if(string.IsNullOrEmpty(User.Password.Name)){
					User.Password.NotValidMessageError = "Password is required";
                    User.Password.IsNotValid = true;
                }else if (User.Password.Name.Length < 5)
				{
					User.Password.NotValidMessageError = "Password must have more than 5 charcteres";
                    User.Password.IsNotValid = true;
                }else{
                    User.Password.IsNotValid = false;
                }

			});
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}