Single-line comment or end-of-line expected
A line with a preprocessor directive cannot have a multiline comment.
The following sample generates CS1025:
#if true /* hello */ // CS1025 #endif // this is a good comment
CS1025 could also occur if you attempt some invalid preprocessor directive:
#define a namespace _1.b { #if a 1 // CS1025, invalid syntax public class a { } #endif public class aa { public static void Main() { } } }