I have a web service that checks a dictionary to see if a file exists and then if it does exist it reads the file, otherwise it saves to the file. This is from a web app. I wonder what is the best way to do this because I occasionally get a FileNotFoundException exception if the same file is accessed at the same time. Here's the relevant parts of the code:

String signature; signature = "FILE," + value1 + "," + value2 + "," + value3 + "," + value4; // this is going to be the filename

        string result;            
        MultipleRecordset mrSummary = new MultipleRecordset();  // MultipleRecordset is an object that retrieves data from a sql server database

        if (mrSummary.existsFile(signature))
        {                
            result = mrSummary.retrieveFile(signature);                
        }
        else
        {                
            result = mrSummary.getMultipleRecordsets(System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString.ToString(), value1, value2, value3, value4);
            mrSummary.saveFile(signature, result);
        }

Here's the code to see if the file already exists:

private static Dictionary dict = new Dictionary();

    public bool existsFile(string signature)
    {                       
        if (dict.ContainsKey(signature))
        {
            return true;
        }
        else
        {
            return false;
        }                                
    }

Here's what I use to retrieve if it already exists:

try {

            byte[] buffer;
            FileStream fileStream = new FileStream(@System.Configuration.ConfigurationManager.AppSettings["CACHEPATH"] + filename, FileMode.Open, FileAccess.Read, FileShare.Read);
              try
              {
                int length = 0x8000;  // get file length
                buffer = new byte[length];            // create buffer
                int count;                            // actual number of bytes read
                JSONstring = "";
                while ((count = fileStream.Read(buffer, 0, length)) > 0)
                {
                    JSONstring += System.Text.ASCIIEncoding.ASCII.GetString(buffer, 0, count);
                }
              }
              finally
              {
                fileStream.Close();
              }
        }
        catch (Exception e)
        {

            JSONstring = "{\"error\":\"" + e.ToString() + "\"}";                
        }

If the file doesn't previously exist it saves the JSON to the file:

try {
if (dict.ContainsKey(filename) == false) { dict.Add(filename, true); } else { this.retrieveFile(filename, ipaddress); }

        }
        catch
        {


        }


        try
        {
            TextWriter tw = new StreamWriter(@System.Configuration.ConfigurationManager.AppSettings["CACHEPATH"] + filename);
            tw.WriteLine(JSONstring);
            tw.Close();
        }
        catch { 

}

Here are the details to the exception I sometimes get from running the above code: System.IO.FileNotFoundException: Could not find file 'E:\inetpub\wwwroot\cache\FILE,36,36.25,14.5,14.75'. File name: 'E:\inetpub\wwwroot\cache\FILE,36,36.25,14.5,14.75' at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy) at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share) at com.myname.business.MultipleRecordset.retrieveFile(String filename, String ipaddress)