Silverlight Handler - Opening a copy of an opened file

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Hi guys, I really need some help or advise.

I have an handler that my Silverlight App uses to deliver a copy of a certain Excel file to the end user in a "Viewing mode":

C#

public class Handler1 : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        string filename = context.Request.QueryString["filename"].ToString();
        string path = context.Request.QueryString["path"].ToString();

        context.Response.AddHeader("Content-Disposition", "attachment; filename=\"" + filename + "\"");
        using (FileStream ficheiro = File.OpenRead(context.Server.MapPath("~/" + path + filename)))
        {
            CopyStream(ficheiro, context.Response.OutputStream);

            context.Response.OutputStream.Flush();
            context.Response.OutputStream.Close();
            context.Response.Flush();
            context.Response.Close();
            context.Response.End();
        }
    }

    public bool IsReusable
    {
        get {return false;}
    }

    private static void CopyStream(Stream input, Stream output)
    {
        byte[] buffer = new byte[32768];
        while (true)
        {
            int read = input.Read(buffer, 0, buffer.Length);
            if (read <= 0)
                return;
            output.Write(buffer, 0, read);
        }
    }
}

This handler is used in the Silverlight app:

C#

System.Windows.Browser.HtmlElement myFrame = System.Windows.Browser.HtmlPage.Document.GetElementById("ifHtmlContent");
if (myFrame != null)
{
    myFrame.SetStyleAttribute("width", "0");
    myFrame.SetStyleAttribute("height", "0");
    myFrame.SetAttribute("src", string.Format("http://mydomain/Handler_Open.ashx?filename={0}&path={1}", FileName, Path));
    myFrame.SetStyleAttribute("visibility", "hidden");
    myFrame.SetStyleAttribute("left", "0");
    myFrame.SetStyleAttribute("top", "0");
}

It works great if the file is closed.

I have another way to open the same Excel file in a "Editing mode" for some users with higher privileges.

No problem here either.

The problem is when one high privilege user has the file opened for editing and another low privilege user tries to open a copy of that file using the above handler. Nothing happens.

I suspect that the File.OpenRead wouldn't work on an opened file but I don't know how to work around it.

Probably my newbiness has something to do with it! :-)

Can anyone advise me on this?

Thanks

Posted 20-Jan-12 6:24am

Jorge J. Martins

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3 solutions

Solution 1

By default, if you keep some file open, you cannot open it again with another file handle by just using its file name; the access will be denied, and for a very good reason. It will happen by default no matter what file or stream related classes are you using: Stream, FileStream, StreamReader, StreamWriter and the like.

You can change this behavior at the level of System.IO.File.Open which returns you an instance of System.IO.FileStream which you can further use immediately or with other classes. Please see:
http://msdn.microsoft.com/en-us/library/y973b725.aspx[^], pay attention for the parameter FileShare share, see http://msdn.microsoft.com/en-us/library/system.io.fileshare.aspx[^].

I would recommend to use it with proper care or avoid it. Shared access is not simple; you need to be very careful as the file stream operations in one thread can disrupt the other.

—SA

Posted 20-Jan-12 7:03am

Sergey Alexandrovich Kryukov

Updated 20-Jan-12 7:04am

v2

Comments

Jorge J. Martins 20-Jan-12 17:39pm

Would it be a solution to make a copy at the OS level and use that copy on the handler?
Can I do that with this type of handler? It's server side, correct?

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Jorge J. Martins · Accepted Answer · 2012-01-20T12:35:00

Yeah! It works.
This is what it looks like after making that copy.
The problem now is that the File.Delete isn't making it's job.
Any clue?

C#

public class Handler1 : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        string filename = context.Request.QueryString["filename"].ToString();
        string caminho = context.Request.QueryString["caminho"].ToString();

        DirectoryInfo TempDir = new DirectoryInfo(context.Server.MapPath("~/TEMP"));
        IEnumerable<fileinfo> FileList = TempDir.EnumerateFiles();

        File.Copy(context.Server.MapPath("~/" + caminho + filename), context.Server.MapPath("~/TEMP/" + filename));
        context.Response.AddHeader("Content-Disposition", "attachment; filename=\"" + filename + "\"");
        using (FileStream ficheiro = File.OpenRead(context.Server.MapPath("~/TEMP/" + filename)))
        {

            CopyStream(ficheiro, context.Response.OutputStream);

            context.Response.OutputStream.Flush();
            context.Response.OutputStream.Close();
            context.Response.Flush();
            context.Response.Close();
            context.Response.End();
            ficheiro.Close();
        }

        File.Delete(context.Server.MapPath("~/TEMP/" + filename));
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

    private static void CopyStream(Stream input, Stream output)
    {
        byte[] buffer = new byte[32768];
        while (true)
        {
            int read = input.Read(buffer, 0, buffer.Length);
            if (read <= 0)
                return;
            output.Write(buffer, 0, read);
        }
    }
}</fileinfo>

Jorge J. Martins · Accepted Answer · 2012-02-27T03:40:00

Got it working a while ago. Had to make it Assync.
Here's how:

C#

public class Handler1 : IHttpAsyncHandler
{
    string fullTempPath;

    public void ServeContent(HttpContext context)
    {
        string filename = context.Request.QueryString["filename"].ToString();
        string caminho = context.Request.QueryString["caminho"].ToString();

        fullTempPath = context.Server.MapPath("~/TEMP/" + filename);

        DirectoryInfo TempDir = new DirectoryInfo(context.Server.MapPath("~/TEMP"));
        IEnumerable<fileinfo> FileList = TempDir.EnumerateFiles();

        string originalFullTempPath = fullTempPath;
        FileInfo fileInfo = new FileInfo(fullTempPath);

            string originalFullTempPathExtension = fileInfo.Extension;

            int i = 1;
            while (File.Exists(fullTempPath))
            {
                fullTempPath = originalFullTempPath.Substring(0, originalFullTempPath.Length - originalFullTempPathExtension.Length) + "[" + i.ToString() + "]" + originalFullTempPathExtension;
                i++;
            }

            File.Copy(context.Server.MapPath("~/" + caminho + filename), fullTempPath);

            context.Response.AddHeader("Content-Disposition", "attachment; filename=\"" + filename + "\"");

            using (FileStream ficheiro = File.OpenRead(fullTempPath))
            {

                CopyStream(ficheiro, context.Response.OutputStream);

                context.Response.OutputStream.Flush();
                context.Response.OutputStream.Close();
                context.Response.Flush();
                context.Response.Close();
                context.Response.End();
                ficheiro.Close();
            }
    }

    private AsyncProcessorDelegate _Delegate;
    protected delegate void AsyncProcessorDelegate(HttpContext context);

    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
        _Delegate = new AsyncProcessorDelegate(ProcessRequest);
        return _Delegate.BeginInvoke(context, cb, extraData);
    }

    public void EndProcessRequest(IAsyncResult result)
    {
        File.Delete(fullTempPath);
        _Delegate.EndInvoke(result);
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

    public void ProcessRequest(HttpContext context)
    {
        ServeContent(context);
    }

    private static void CopyStream(Stream input, Stream output)
    {
        byte[] buffer = new byte[32768];
        while (true)
        {
            int read = input.Read(buffer, 0, buffer.Length);
            if (read <= 0)
                return;
            output.Write(buffer, 0, read);
        }
    }
}</fileinfo>

Silverlight Handler - Opening a copy of an opened file

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