#if #elseif #endif

#if, #elseif, and #endif are preprocessor directives commonly used in programming languages such as C, C++, and C#. These directives are used to conditionally compile code based on certain conditions.

The #if directive allows you to test a condition and include or exclude code based on the result. It is followed by a condition, which can be a defined constant, a macro expression, or a combination of these. If the condition evaluates to true, the block of code following the #if directive is compiled. Otherwise, it is skipped.

The #elseif directive is used to test an additional condition if the preceding #if or #elseif condition(s) evaluated to false. It provides an alternative condition to be checked. If the condition evaluates to true, the block of code following the #elseif directive is compiled. Otherwise, it is skipped.

The #endif directive marks the end of a conditional block. It is used to close the block of code associated with the most recent #if or #elseif directive.

Here's an example usage of these directives in C:

#define DEBUG_MODE

#if defined(DEBUG_MODE)
    printf("Debug mode is enabled.\n");
#elif defined(TEST_MODE)
    printf("Test mode is enabled.\n");
#else
    printf("No special mode is enabled.\n");
#endif

In this example, if the `DEBUG_MODE` macro is defined, the code within the first block will be compiled and executed. If not, it will check for the `TEST_MODE` macro and execute the code within the second block if it is defined. If neither macro is defined, the code within the else block will be executed.