Question

In my application the user can drag and drop multiple text files onto a GUI control to convert them to another format. Here is the relevant code:

    private void panelConverter_DragDrop(object sender, DragEventArgs e)
    {
        string[] filenames = (string[])e.Data.GetData(DataFormats.FileDrop);
        foreach (string filename in filenames)
        {
            convertFile(filename);
        }
    }

    private void convertFile(string filename)
    {
        // build name of output file
        string convertedFile = Path.ChangeExtension(filename, ".out");

        // open input file for reading
        FileInfo source = new FileInfo(filename);
        StreamReader srcStream = source.OpenText();

        // open output file for writing
        StreamWriter dstStream = new StreamWriter(convertedFile);

        // loop over input file
        string line;
        do
        {
            // get next line from input file
            line = srcStream.ReadLine();

            if (!Regex.IsMatch(line, @"fred=\d+"))
            {
                dstStream.WriteLine(line);
                dstStream.Flush();
            }
        } while (line != null);
    }

The problem is that when I drop multiple files on the GUI, only one of them actually gets processed. I have found that if I comment out the Regex line, all of the dropped files are processed. Am I missing something in my handling of regular expressions in this context?

Answer 1

Try following variation of the method:

private void convertFile(string filename)
{
    // build name of output file
    string convertedFile = Path.ChangeExtension(filename, ".out");

    // open input file for reading
    FileInfo source = new FileInfo(filename);
    StreamReader srcStream = source.OpenText();

    // open output file for writing
    using (StreamWriter dstStream = File.CreateText(convertedFile))
    {
        // loop over input file
        string line;
        do
        {
            // get next line from input file
            line = srcStream.ReadLine();

            if (!Regex.IsMatch(line, @"fred=\d+"))
            {
                dstStream.WriteLine(line);
                dstStream.Flush();
            }
        } while (line != null);
    }

    Debug.WriteLine(string.Format("File written to: {0}", convertedFile));
}

The main modification is use of using keyword which would guarantee disposal and closing of the file resource. If problem is not still resolved then try followings:

• Do you have any global exception handlers? Make sure you check Debug > Exceptions… so that Visual Studio automatically breaks on the line where exception is thrown. See this article on how-to.
• Make sure files are written at correct places. If files have full path then the Debug.WriteLine statement above would tell you were the files are being written.

You should get at least 0 length file written on the disk if no exceptions are occurring.