In the command prompt, an easy way to append files together is to use the "type" command and pipe it to an output file.
type Results\_\* > Results\_combined.txt
To call this from a piece of C# code, I have been using the command line with the /c parameter, which makes it execute the attached command.
cmd /c type Results\_\* > Results\_combined.txt
C# Example usage:
using (Process RunProc = new Process()) {
string s = "cmd.exe";
string p = "/c type Results\_\* > Results\_combined.txt";
RunProc.StartInfo = new ProcessStartInfo(s, p);
RunProc.Start();
RunProc.WaitForExit();
}