how to merge all avi file in a folder using c#

See more:

C#

ASP.NET

I am working on a project where I need to merge avi files. I got help from this link:

But in my case I have a folder that contains several avi files

I want to merge all these.

When I merged ist two files and after that it is giving me error like "An invalid media type was specified"

Here is my code block.

foreach (string fileName in System.IO.Directory.GetFiles(LPath)) {

        using (ITimeline timeline = new DefaultTimeline())
        {
            IGroup group = timeline.AddVideoGroup(32, 720, 576);

            var firstVideoClip = group.AddTrack().AddVideo(downloadedfilename);
            var secondVideoClip = group.AddTrack().AddVideo(fileName, firstVideoClip.Duration);

            using (AviFileRenderer renderer = new AviFileRenderer(timeline, downloadedfilename))
            {
                renderer.Render();
            }
        }
    }  

my this line of code giving error:

var firstVideoClip = group.AddTrack().AddVideo(downloadedfilename);

can anybody help me to merge all files in the folder.