How to merge two or more files using C#

Few days ago i have a requirement to make a processing engine which will download, decode & staging almost 70k+ ASN files. So that i have developed an ASN Decoder using Turbo C. Then make a DLL to use this decoder in my C# project. Everything is good but one problem is that my processing exe will take almost 6 hours to process all files. After reviewing my source code i found that each time my