How To Read And Write Text File

I want to read and write a txt file but

The process cannot access the file 'D:BelgelerimVisual Studio 2005WebSitesmacrotechCounter.txt' because it is being used by another process.

what is wrong with this

StreamWriter FileWriter;
StreamReader FileReader;
string Countstr;
string str1;
int Len1,i;
FileReader = File.OpenText(Server.MapPath(Page.ResolveUrl("~/Counter.txt")));
FileReader = File.OpenText(Server.MapPath("~/Counter.txt"));
Countstr = FileReader.ReadLine();
FileReader.Close();
FileReader.Dispose();
i = Convert.ToInt32(Countstr) + 1;
Countstr = Convert.ToString(i);
FileWriter = File.CreateText(Server.MapPath("~/Counter.txt"));
FileWriter.WriteLine(Countstr);
FileWriter.Close();