You are writing to the file prior to closing your filestream:
using(FileStream fs=new FileStream(path,FileMode.OpenOrCreate))
using (StreamWriter str=new StreamWriter(fs))
{
str.BaseStream.Seek(0,SeekOrigin.End);
str.Write("mytext.txt.........................");
str.WriteLine(DateTime.Now.ToLongTimeString()+" "+DateTime.Now.ToLongDateString());
string addtext="this line is added"+Environment.NewLine;
str.Flush();
}
File.AppendAllText(path,addtext); //Exception occurrs ??????????
string readtext=File.ReadAllText(path);
Console.WriteLine(readtext);
The above code should work, using the methods you are currently using. You should also look into the using statement and wrap your streams in a using block.