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C# code for if you are trying to get the mp3 length or duration of a media file.  You will need to reference Windows Media Player.  Go to Com Add-ins to add the wmp.dll to your project. 

string Duration = null;
WMPLib.WindowsMediaPlayer w = new WMPLib.WindowsMediaPlayer();
WMPLib.IWMPMedia m = w.newMedia(Filename);
if (m != null) {
 Duration = m.durationString;
}
w.close();