Writing in files ASP.NET C# and NOT locking them afterwards

Question

I am getting this error: The process cannot access the file (...) because it is being used by another process. I have tried to use File.WriteAllText;

StreamWriter sw = new StreamWriter(myfilepath);
                sw.Write(mystring);
                sw.Close();
                sw.Dispose();

;

using (FileStream fstr = File.Create(myfilepath))
                {
                StreamWriter sw = new StreamWriter(myfilepath);
                sw.Write(mystring);
                sw.Close();
                sw.Dispose();
                fstr.Close();
                }

All I am trying to do is to access a file, write on it, then close it. I might be making a silly mistake but I would like to understand what I am doing wrong and why. How to make sure that the file is closed and not to cause this error again.

Helped by the answers so far I did this:

using (FileStream fstr = File.Open(myfilepath,FileMode.OpenOrCreate,FileAccess.ReadWrite))
                {
                    StreamWriter sw = new StreamWriter(fstr);
                    sw.Write(mystring);
                    sw.Close();


                }

It seems to be better because it seems to close/stop the process of my file if I try to access another file on the second time I access the page. But if I try to access the same file on a second time, it gives me the error again.

Answer 1

Why not just use:

System.IO.File.WriteAllText(myfilepath, mystring");

That should not lock your file.

Internally WriteAllText uses FileShare.Read and releases that lock as soon as it is done writing.