Program:

The source code to demonstrate the #error preprocessor directive is given below. The given program is compiled and executed successfully on Microsoft Visual Studio.

//C# program to demonstrate the 
//#error preprocessor directive.

#define MACRO1
#define MACRO2
#define MACRO3
#undef  MACRO2

using System;

class Program
{
   public static void Main()
   {
        #if (!MACRO2)
            #error MACRO2 is not defined
        #endif
    }
}

Output:

Error	1	#error: 'MACRO2 is not defined'	

Explanation:

The above program will generate a syntax error because here we used the #error directive with an error message in the Main() method then the specified message will print as a syntax error when we compile the program.