C#: System.Diagnostics.Process.Start("Explorer.exe", @"/select" + FilePath). Can not open file when file's name is unicode character

How are you getting the "FilePath" value? You need to provide the Unicode characters, correct? Are you doing that? For example, if you are entering the name using a TextBox then you must be able to enter the Unicode characters into it. On that subject, I am not sure if the font used for the form with the text box is relevant. If the font is not a Unicode font then that is a likely problem and I don't know if Windows Forms always uses Unicode fonts. As far as I know, there are few Unicode fonts in most systems.

Also, are you sure that the file path (including name and extension) is 260 characters or less? If not then it will not work unless you use Universal Naming Convention (UNC) names.

Also note that questions in forums won't work if you say things like "could not" without an explanation. What does happen? Your question might work much better (you might get an answer) if you clarify what "could not" means.

Finally, try the following for diagnostic purposes. Go to the folder with the file (one of them) using the Windows Explorer. Go to the folder's properties and copy the path and paste that into a C# program. If you need to, you can create a test program in less than a minute. Then do the same for the file's name and put all that in a string constant with a backslash and all to make a complete path. Then try opening Windows Explorer with that. If that "works" then you know it is possible and you just need to figure out how to get the correct path into your program. If it does not work then you have significantly limited the direction you need to go to diagnose the problem.

I know but, if you could try it with a very short file name, it will tell us whether there's any merit in this suggestion which is the only one I have :)

Otherwise, I'd wait for more answers.

It is not an easy thing to do like that...