How to use #if..#elif...#else...#endif directives in C#?

Preprocessor directives in C# begin with # and are processed before compilation. The #if..#elif...#else...#endif directives allow conditional compilation, meaning you can include or exclude code blocks based on defined symbols.

These directives are useful for creating debug builds, platform-specific code, or feature toggles without affecting runtime performance.

Syntax

Following is the syntax for conditional compilation directives −

#if condition
   // code block 1
#elif condition
   // code block 2
#else
   // code block 3
#endif

Key Directives

• #if − Tests if a symbol evaluates to true

• #elif − Allows multiple conditional branches (else-if)

• #else − Executed when all previous conditions are false

• #endif − Marks the end of the conditional block

Using #if with Single Symbol

Example

#define DEBUG

using System;

class Program {
   static void Main(string[] args) {
      Console.WriteLine("Application started");
      
      #if DEBUG
      Console.WriteLine("Debug mode is ON");
      #endif
      
      Console.WriteLine("Application finished");
   }
}

The output of the above code is −

Application started
Debug mode is ON
Application finished

Using #if..#elif..#else..#endif

Example

#define RELEASE
#undef DEBUG

using System;

class Program {
   static void Main(string[] args) {
      #if DEBUG
      Console.WriteLine("Debug version");
      #elif RELEASE
      Console.WriteLine("Release version");
      #else
      Console.WriteLine("Unknown version");
      #endif
      
      Console.WriteLine("Program executed successfully");
   }
}

The output of the above code is −

Release version
Program executed successfully

Using Multiple Conditions with Logical Operators

Example

#define WINDOWS
#define X64

using System;

class Program {
   static void Main(string[] args) {
      #if (WINDOWS && X64)
      Console.WriteLine("Windows 64-bit platform");
      #elif (WINDOWS && !X64)
      Console.WriteLine("Windows 32-bit platform");
      #elif (!WINDOWS && X64)
      Console.WriteLine("Non-Windows 64-bit platform");
      #else
      Console.WriteLine("Non-Windows 32-bit platform");
      #endif
   }
}

The output of the above code is −

Windows 64-bit platform

Common Use Cases

Conclusion

The #if..#elif..#else..#endif directives provide conditional compilation in C#, allowing you to include or exclude code blocks based on defined symbols. These directives are processed at compile-time and are essential for creating platform-specific builds, debug configurations, and feature toggles.