Have you tried
string fileName = @"D:\Selected Instances\Insertion graphs\1-FullIns_3.col";
string text = System.IO.File.ReadAllText(fileName);
The @ character cannot be used with a variable, only literal strings.
See
String Class (System)[
^]
Quote:
Note that in C#, because the backslash (\) is an escape character, literal backslashes in a string must be escaped or the entire string must be @-quoted.