Question

how do I use a file that is currently being used by another process?

Answer 1

If the file is opened with sharing permissions then you should just be able to use it. On the other hand if the process opened the file with non-sharing permissions you cannot access it until the process in question releases the file.

If you own the processes in question, you can enable read sharing by opening the file in the following way.

using (var file = new FileStream(    @'C:\path\to\file.txt',     FileMode.Open,     FileAcces.Read,     FileShare.Read) {   // ... } 

The important parameter there is the FileShare.Read