C# 如何使用Environment.SpecialFolder在Xamarin/VisualStudio项目中指定特定文件？

我试图将文本附加到文本文件中，但我找不到任何地方如何实际定位现有文件

 partial void MainBtn_TouchUpInside(UIButton sender)
        {
            var Pp = ("Selected Form Of Exchange: Paypal");
            string mydocpath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments(WHAT DO I WRITE IN THIS SPACE??);
            using (StreamWriter outputFile = new **StreamWriter(Path.Combine(mydocpath, "SelectPayment.txt")))

//我知道文件已经存在，我只是在尝试不同的事情

            {

                outputFile.WriteLine(Pp);

}

那我该怎么办？谢谢

---

Josh

通过组合文件夹路径和文件名来创建文件路径，从而指定文件

// this returns the path to a folder
string path = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments));

// combine that with a file name to create a file path
string file = Path.Combine(path, "myFile.text");

// append text to file
File.AppendAllText(file, "this is the text I want to append);