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C# Fundamentals - Pre-Processor Directives

C# Fundamentals - Pre-Processor Directives

The pre-processor directives give instruction to the compiler to process some information before the actual compilation starts.
All pre-processor directives begin with #, and only white-space characters may appear before a pre-processor directive on any given line.
As pre-processor directives are not statements, they do not require a semi-colon (;) to the placed at the end of the line.
C# compiler does not have a separate pre-processor; however, the directives are processed as if there was one. In C# the pre-processor directives are used to help in conditional compilation.
Unlike C and C++ directives, they are not used to create macros. So as they are less flexible in what they can do in C++; they are mostly used for conditional compilation ( whether a set of lines of code would be compiled or not ). A pre-processor directive must be the only instruction on a line.
There are 6 main directives used in C#, they are:
  • #if
  • #else
  • #elif
  • #endif
  • #define
  • #undef
#if, #else, #elif & #endif
using System;
public class TestClass
{
    static void Main()
    {
#if (DEBUG && !BITSHIFT)
        Console.WriteLine("DEBUG is defined");
#elif (!DEBUG && BITSHIFT)
        Console.WriteLine("BITSHIFT is defined");
#elif (DEBUG && BITSHIFT)
        Console.WriteLine("DEBUG and BITSHIFT are defined");  
#else
        Console.WriteLine("DEBUG and BITSHIFT are not defined");
#endif
    }
}
DEBUG & BITSHIFT are not defined
As you can see, the DEBUG & BITSHIFT symbol is not defined.
#define & undef
#define BITSHIFT
#define DEBUG
#undef DEBUG
using System;
public class TestClass
{
    static void Main()
    {
#if (DEBUG && !BITSHIFT)
        Console.WriteLine("DEBUG is defined");
#elif (!DEBUG && BITSHIFT)
        Console.WriteLine("BITSHIFT is defined");
#elif (DEBUG && BITSHIFT)
        Console.WriteLine("DEBUG and BITSHIFT are defined");  
#else
        Console.WriteLine("DEBUG and BITSHIFT are not defined");
#endif
    }
}
BITSHIFT is defined
Now we have defined BITSHIFT but DEBUG was not. Even though we defined it, we undefined right after that.
In a practical use-case scenario you would define compiler options for these defines and you might want to undefine those compiler set define statements, That's where we use 'undef'.
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