private void button1_Click(object sender, EventArgs e)
{
     System.IO.File.WriteAllText("F:\Learn\windows\form", "به نام خدا");
}   

private void button1_Click(object sender, EventArgs e)
{
     System.IO.File.WriteAllText("F:\Learn\windows\form\نام برای فایل"","به نام خدا");
}   

private void button1_Click(object sender, EventArgs e)
{
     System.IO.File.WriteAllText(@"F:\Learn\windows\form\نام برای فایل"","به نام خدا");
}   

private void button1_Click(object sender, EventArgs e)
{
     System.IO.File.WriteAllText(@"F:\Learn\windows\form\نام برای فایل"",نام کادر متن.text);
}   

private void button1_Click(object sender, EventArgs e)
{
     System.IO.File.WriteAllText(SaveFileDialog1.FileName,نام کادر متن.text);
}   

private void button2_Click(object sender, EventArgs e)
{
      if (openFileDialog1.ShowDialog() == DialogResult.OK)
      {
            txtmatn.text = System.IO.File.ReadAllText(openFileDialog1.FileName);
      }
}

private void button3_Click(object sender, EventArgs e)
{
     if (openFileDialog1.ShowDialog() == DialogResult.OK)
     {
       نام کادرمتن.text=System.IO.File.ReadAllText(openFileDialog1.FileName,txtapp.tex);
     }
}