Android Write and Read Text File in C#

I want to write a code in C# that reads a text file in Android and if it doesn't exist, create it. I want my application to read the same file everytime I run my App and create that file the first time I open my app.I wrote this code but it doesn't work.It only creates a directory if it doesn't exist but it doesn't create my text file.Thank you for your answer in advance.

string rootPath = Android.App.Application.Context.GetExternalFilesDir(null).ToString();
var filePathDir = Path.Combine(rootPath, "PhoenixWallet/");
if (!File.Exists(filePathDir)) {
Directory.CreateDirectory(filePathDir);
}
bitcoinPrivateKeysFile = filePathDir + "bitcoinwallets.phoenixbtcwallet";
string bitcoinprivateKeyItem;
if (File.Exists(bitcoinPrivateKeysFile) != null) {
StreamReader sr = File.OpenText(bitcoinPrivateKeysFile);
while (!sr.EndOfStream) {
bitcoinprivateKeyItem = sr.ReadLine();
bitcoinprivKeys.Add(bitcoinprivateKeyItem);
}
Console.WriteLine("File Exists");
sr.Close();
} else {
Key bitcoinKey = new Key();
BitcoinSecret bitcoinsecret = new BitcoinSecret(bitcoinKey, network);
StreamWriter sw;
sw = File.CreateText(bitcoinPrivateKeysFile);
sw.WriteLine(bitcoinsecret.PrivateKey);
Console.WriteLine("File Does not Exist");
sw.Close();
}