das

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            DataContext = new Person();
        }
    }

    class Person : IDataErrorInfo
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Email { get; set; }
        public string this[string columnName]
        {
            get
            {
                if(columnName == nameof(FirstName))
                {
                    if(string.IsNullOrWhiteSpace(FirstName))
                    {
                        return "Pole imie jest wymagane";
                    }
                }

                return string.Empty;
            }
        }

        public string Error { get; }
    }
}