view.xaml

<Window
    x:Class="WpfApp3.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:vm="clr-namespace:WpfApp3.ViewModels"
    Title="File comparer"
    Width="800"
    Height="450">

    <Window.DataContext>
        <vm:ApplicationViewModel />
    </Window.DataContext>

    <Grid>
        <Button
            x:Name="button"
            Width="57"
            Margin="10,87,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Command="{Binding BrowseFolder}"
            CommandParameter="0"
            Content="Folder 1" />
        <Button
            x:Name="button_Copy"
            Width="66"
            Height="19"
            Margin="683,87,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Command="{Binding BrowseFolder}"
            CommandParameter="1"
            Content="Folder 2" />
        <TextBox
            x:Name="TextBox"
            Width="192"
            Height="18"
            Margin="72,87,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            SelectionOpacity="1"
            SpellCheck.IsEnabled="True"
            Text="{Binding TextBox1}"
            TextWrapping="Wrap" />
        <TextBox
            x:Name="textBox_Copy"
            Width="168"
            Height="17"
            Margin="498,88,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Text="{Binding TextBox2}"
            TextWrapping="Wrap" />
        <TextBlock
            x:Name="textBlock_Copy1"
            Width="254"
            Height="248"
            Margin="10,142,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Background="Beige"
            Text="{Binding TxtBlock}"
            TextWrapping="Wrap" />
        <Button
            x:Name="button1"
            Width="75"
            Margin="436,35,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Content="Compare" />
        <TextBlock
            x:Name="textBlock_Copy"
            Width="251"
            Height="236"
            Margin="498,124,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Background="Beige"
            Text="{Binding TxtBlock}"
            TextWrapping="Wrap" />
    </Grid>

</Window>

vm.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Input;
using WpfApp3.Model;
using Winforms = System.Windows.Forms;

namespace WpfApp3.ViewModels
{
    public class ApplicationViewModel : INotifyPropertyChanged
    {
        private UserFile _selectedFile;

        public UserFile SelectedFile
        {
            get => _selectedFile;
            set
            {
                _selectedFile = value;

                OnPropertyChanged("SelectedFolder");
            }
        }

        private string textBox1;

        public string TextBox1
        {
            get => textBox1;
            set
            {
                textBox1 = value;
                OnPropertyChanged(nameof(TextBox1));
            }
        }

        private string textBox2;

        public string TextBox2
        {
            get => textBox2;
            set
            {
                textBox2 = value;
                OnPropertyChanged(nameof(TextBox2));
            }
        }

        private string txtBlock;

        public string TxtBlock
        {
            get => txtBlock;
            set
            {
                txtBlock = value;
                OnPropertyChanged(nameof(TxtBlock));
            }
        }

        public ICommand Compare => new RelayCommand((obj) =>
                 {
                     try
                     {
                         IEnumerable<string> oldFiles = Directory.EnumerateFiles("TextBox1").Select(Path.GetFileName);
                         IEnumerable<string> newFiles = Directory.EnumerateFiles("TextBox2").Select(Path.GetFileName);
                         newFiles = newFiles.Except(oldFiles);
                     }
                     catch (IOException e)
                     {
                         MessageBox.Show(e.ToString());
                     }
                 });
        public ApplicationViewModel()
        {
            BrowseFolder = new RelayCommand(Browse);
        }
        public void Browse(object obj)
        {
            if (obj is string ival)
            {
                Winforms.FolderBrowserDialog fbd = new Winforms.FolderBrowserDialog
                {
                    RootFolder = Environment.SpecialFolder.DesktopDirectory,
                    Description = "Select Folder",
                    ShowNewFolderButton = false
                };
                if (fbd.ShowDialog() == Winforms.DialogResult.OK)
                {
                    if (ival == "0")
                    {
                        TextBox1 = fbd.SelectedPath;
                    }
                    else if (ival == "1")
                    {
                        TextBox2 = fbd.SelectedPath;
                    }
                    var files =
                        Directory.GetFiles(fbd.SelectedPath)
                        .Select(file => Path.GetFileName(file));

                    TxtBlock = string.Join("\n", files);
                }
            }

        }

        public ICommand BrowseFolder { get; }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string prop = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(prop));
        }
    }
}