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Compiler Error CS1025 

Error Message

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, as follows:

// CS1025.cs
#define a

class Sample
   static void Main()
      #if a 1   // CS1025, invalid syntax
         System.Console.WriteLine("Hello, World!");