MainWindow.xaml:

<Window x:Class="ImageToBase64WPF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ImageToBase64WPF"
        mc:Ignorable="d"
        WindowStartupLocation="CenterScreen"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition></RowDefinition>
                <RowDefinition Height="auto"></RowDefinition>
            </Grid.RowDefinitions>
            <Image x:Name="ingSource"></Image>
            <Button Grid.Row="1" Content="Open" Click="btnOpen_Click" x:Name="btnOpen"></Button>
        </Grid>
        <Grid Grid.Column="1">
            <Grid.RowDefinitions>
                <RowDefinition></RowDefinition>
                <RowDefinition Height="auto"></RowDefinition>
            </Grid.RowDefinitions>
            <Button Grid.Row="1" Click="btnSave_Click" Content="Save" x:Name="btnSave"></Button>
            <TextBox TextWrapping="Wrap" x:Name="txtMutiplineText"></TextBox>
        </Grid>
    </Grid>
</Window>

MainWindow.xaml.cs:

using Microsoft.Win32;
using System;
using System.Collections.Generic;
using System.Drawing;
using System.IO;
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 ImageToBase64WPF
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        string Base64Text;
        private void btnOpen_Click(object sender, RoutedEventArgs e)
        {
            OpenFileDialog dialog = new OpenFileDialog();
            dialog.Filter = "Image Files (* .BMP;* .JPG;* .PNG)|* .BMP;* .JPG;* .PNG" + "|All files (*.*)|*.*";
            dialog.CheckFileExists = true;
            dialog.Multiselect = false;
            if (dialog.ShowDialog() == true)
            {
                Bitmap image = new Bitmap(dialog.FileName);
                ingSource.Source = new BitmapImage(new Uri(dialog.FileName));
                byte[] ImageArray = System.IO.File.ReadAllBytes(dialog.FileName);
                Base64Text = Convert.ToBase64String(ImageArray);
                txtMutiplineText.Text = Base64Text;
            }
        }

        private void btnSave_Click(object sender, RoutedEventArgs e)
        {
            string path = "ImageToBase64.txt";
            using (StreamWriter stream = File.CreateText(path))
            {
                stream.Write(Base64Text);
            }
        }
    }
}

Em lưu code này lại để sau này sử dụng. Mong admin xét duyệt. Em xin cảm ơn