Error: File is being used by another process in ASP.Net

Hi,

I have a project with C# ASP.Net. I should create a web site which has categories.There will be a button and textbox and when I click this button there will be created a new web page and button and what I wrote text box it will be new button's text. At the same time, this created button's text will saved at SQL Server

I tried a create new button on page load but when I execute the project the button was created but I want to create with another button.These are for buttons; 

and these are for a new web page;(but there is an error -->Sayac.txt is already using)

sayac =int.Parse(rdr.ReadLine())+1;
File.Copy(MapPath("_template.aspx"), MapPath("WebForm" + sayac + ".aspx"));
rdr.Close();
wrt = new StreamWriter(MapPath("sayac.txt"), false);
wrt.WriteLine(sayac);
wrt.Close();