using System.IO;

ファイル上書き
File.WriteAllText(@”C:\Myfolder\test.txt”, “Good morning!”);
追記
File.AppendAllText(@”C:\Myfolder\test.txt”, “Good morning!”);

読み込み
if (!File.Exists(path))
{
string readText = File.ReadAllText(@”C:\Myfolder\test.txt”);}

}