XceedBinaryEncodingLib.XceedBinaryEncoding encoder = new XceedBinaryEncodingLib.XceedBinaryEncodingClass(); 

encoder.License( @"your license key" );      

try
{
  encoder.EncodingFormat = new XceedBinaryEncodingLib.XceedBase64EncodingFormatClass(); 

  object bytesRead = null;

  // Encode the file
  encoder.ProcessFile( @"c:\test\file.txt", 0, 0, 
                       XceedBinaryEncodingLib.EXBFileProcessing.bfpEncode, true,
                       @"c:\test\encoded.txt", false, ref bytesRead ); 

  // Decode the file
  encoder.ProcessFile( @"c:\test\encoded.txt", 0, 0,
                       XceedBinaryEncodingLib.EXBFileProcessing.bfpDecode, true, 
                       @"c:\test\decoded.txt", false, ref bytesRead );       
}
catch( System.Runtime.InteropServices.COMException except )
{
  MessageBox.Show( except.ToString() );
}