Microsoft Certified Solutions Developer (MCSD) Certification Practice Test 2026 - Free MCSD Practice Questions and Study Guide

The preprocessor directive used to define a symbol in C# is the `#define` directive. When you use `#define`, you are essentially creating a symbolic name that can be used throughout your code. This is particularly useful for enabling or disabling sections of code conditionally based on whether the symbol is defined or not.

For instance, if you define a symbol using `#define DEBUG`, you can later check if that symbol is defined using the `#if` directive. This allows for flexibility in managing code for different build configurations, such as debug versus release setups.

The other directives mentioned serve different purposes. For instance, `#undef` is used to undefine a symbol, making it unavailable for further use in the code. `#if` is used to include sections of code conditionally based on whether a specific symbol has been defined, while `#pragma` is used for compiler-specific instructions and does not define symbols. Thus, the use of `#define` is crucial for controlling code compilation and maintenance in C#.