private void button2_Click(object sender, EventArgs e)
        {
            button1.Enabled = true;
            
           
                OpenFileDialog dlgOpen = new OpenFileDialog();
                dlgOpen.Title = "Select one or more files";
                dlgOpen.InitialDirectory = "d:\\";
                dlgOpen.Filter = "txt files (*.txt|*.txt";
                dlgOpen.FilterIndex = 2;
                dlgOpen.RestoreDirectory = true;
                dlgOpen.ShowReadOnly = true;
                dlgOpen.Multiselect = true;
                if (dlgOpen.ShowDialog() == System.Windows.Forms.DialogResult.OK)
                {
                    foreach (string fileName in dlgOpen.FileNames)
                    {
                    sr = new StreamReader(dlgOpen.FileName);
                    StreamWriter tw = File.AppendText(@"d:\Test3.txt");
                    tw.WriteLine();
                    tw.WriteLine(sr.ReadToEnd());
                    tw.WriteLine(DateTime.UtcNow);
                    tw.Flush();
                    tw.Close();
                    sr.Close();
                    button1.Enabled = false;
                    
                   
                    } 
                    
                }
            }

sr = new StreamReader(dlgOpen.FileName);

sr = new StreamReader(fileName);