public partial class Form1 : Form
    {
         public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBox1.Text = "";
            try
            {
                using (var mmf = MemoryMappedFile.OpenExisting("Test"))
                using (var stream = mmf.CreateViewStream())
                using (StreamReader sr = new StreamReader(stream))
                {
                    while (!sr.EndOfStream)
                    {
                        textBox1.Text = sr.ReadLine();
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }