public void bt_Click(Object sender, EventArgs e)
{
    if (sender == bt1)
    {
        OpenFileDialog ofd = new OpenFileDialog();
        ofd.Filter = "テキストファイル|*.txt";

        if (ofd.ShowDialog() == DialogResult.OK)
        {
            StreamReader sr =
                new StreamReader(ofd.FileName, 
                    System.Text.Encoding.Default);
            tb.Text = sr.ReadToEnd();
            sr.Close();
        }
    }
    else if (sender == bt2)
    {
        SaveFileDialog sfd = new SaveFileDialog();
        sfd.Filter = "テキストファイル|*.txt";

        if (sfd.ShowDialog() == DialogResult.OK)
        {
            StreamWriter sw 
                = new StreamWriter(sfd.FileName);
            sw.WriteLine(tb.Text);
            sw.Close();
        }
    }
}