string textBoxesValues = "TextBox1.Text" + "," + "TextBox2.Text" + "," + "TextBox3.Text" + "," + "TextBox4.Text" + "," + "TextBox5.Text";
StreamWriter writer = new StreamWriter("FilePath where you want to save e.g D:\\New Folder\abc.text", true);
writer.WriteLine(textBoxesValues);
writer.Flush();
writer.Close();
writer.Dispose();
File.AppendAllText("FilePath where you want to save e.g D:\\New Folder\abc.text", textBoxesValues);
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