Preprocessor Directives #

The preprocessor in C# is much simpler than in C/C++.

• There is no #include directive (in C#, we use using to import namespaces).
• #define does not allow creating macros with values or functional macros. It is only used for defining conditional symbols.

Conditional Compilation (#if, #elif, #else, #endif) #

This is the most common and important use of directives. It allows compiling blocks of code only when a specific symbol is defined. Symbols can be defined and undefined using the #define and #undef directives.

Logical operators !, &&, and || can be used within conditions.

#define TEST
#undef TRACE
#if TEST
    Console.WriteLine("TEST is defined");
#endif
#if !TEST
    Console.WriteLine("TEST is not defined");
#endif
#if TEST && DEBUG
    Console.WriteLine("DEBUG and TEST is defined");
#else
    Console.WriteLine("DEBUG and TEST are not both defined");
#endif

They can also be defined in the project file. Some symbols, such as DEBUG, are automatically included, for example, in the Debug configuration.

<PropertyGroup>
    <DefineConstants>TRACE;DEMO</DefineConstants>
</PropertyGroup>

Nullable Context #

The #nullable directive can be used to enable and disable the nullable context, in which the compiler performs static analysis for type nullability.

#nullable disable
string str1 = null;
#nullable enable
string str2 = null!;
#nullable restore
string str3 = null!;

Regions #

Regions themselves do nothing. They group code into blocks that can be collapsed in the IDE.

#region REGION_EXPLANATION
private static void Region()
{
    PrintCurrentMethodName();
    Console.WriteLine("""
        region is a preprocessor directive
        that is used to group related pieces
        of code together in a way that can
        be collapsed or expanded in the code editor.
        """);
}
#endregion

Warning Control #

The #pragma warning directive allows suppressing and restoring compiler warnings.

#pragma warning disable CS0219 // unused variable
string str = "Hello, world!";
#pragma warning restore CS0219

Error codes can be found in the GenerateCSharpErrors repository.

Directives can also be used to generate compilation warnings and errors, which is useful in conjunction with conditional compilation:

#if !DEMO
    #error DEMO is not defined
    Console.WriteLine("This code will not "+
    "compile if DEMO is not defined.");
#else
    #warning DEMO is defined
    Console.WriteLine("This code will generate a warning message if DEMO is defined.");
#endif