How to use #ifdef and #ifndef to check if an identifier has been defined

#ifdef#ifndef and #endif are preprocessor directives which allow us to check wether or not a value has already been defined using the #define directive. This can be useful when you’re including files that may already have the same value defined using #define.

Here’s an example that defines the value Whidbey only if it wasn’t defined before:

#ifndef Whidbey
#define Whidbey

Similarly, what’s between #ifdef and #endif is compiled only if #ifdef returns true (and the value was defined before):

#ifdef Whidbey
// Whidbey is already defined
Nathan Pakovskie is an esteemed senior developer and educator in the tech community, best known for his contributions to With a passion for coding and a knack for simplifying complex tech concepts, Nathan has authored several popular tutorials on C# programming, ranging from basic operations to advanced coding techniques. His articles, often characterized by clarity and precision, serve as invaluable resources for both novice and experienced programmers. Beyond his technical expertise, Nathan is an advocate for continuous learning and enjoys exploring emerging technologies in AI and software development. When he’s not coding or writing, Nathan engages in mentoring upcoming developers, emphasizing the importance of both technical skills and creative problem-solving in the ever-evolving world of technology. Specialties: C# Programming, Technical Writing, Software Development, AI Technologies, Educational Outreach

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