Preprocessor directives

Tip

New to developing software? You won't need preprocessor directives right away. Focus on the Get started tutorials first and come back here when your projects require conditional compilation or build configuration.

Experienced in another language? If you're familiar with #ifdef in C/C++ or conditional compilation in other languages, C# preprocessor directives work similarly. Skim ahead to the syntax you need.

C# preprocessor directives tell the compiler what code to include, exclude, or treat differently when it builds your app. This guidance can change the resulting program. Preprocessor directives always start with # and must appear on their own line (ignoring leading whitespace). You can add a trailing comment after the directive. While the language reference documents all available directives, three groups cover everyday use:

• File-based apps (#:) - configure file-based apps.
• Conditional compilation (#if / #elif / #else / #endif) — include or exclude code based on build configuration or target framework.
• Warning suppression (#pragma warning) — suppress or restore specific compiler warnings.

File-based app directives

Starting with C# 14, file-based apps use two additional directives:

• #! — the shebang line that enables executing the file directly on Unix (for example, ./Program.cs). This requires the execute permission to be set on the file (chmod +x <file>).
• #: — build-system directives that configure packages, SDK settings, and other options for single-file programs.

Use #:package to add a NuGet package. For example, the following file-based app uses the Spectre.Console package to render styled output:

#!/usr/bin/env dotnet
#:package Spectre.Console@*

AnsiConsole.MarkupLine("[bold green]Hello[/] from a file-based app!");

You can specify an exact version with @, or use @* to pull the latest version. Add multiple #:package directives to include more packages:

#:package Serilog@3.1.1

Other #: directives let you reference projects, set MSBuild properties, or change the SDK:

#:project ../SharedLibrary/SharedLibrary.csproj
#:property PublishAot=false
#:sdk Microsoft.NET.Sdk.Web

For the full list of directives, see File-based apps and the language reference.

Conditional compilation

Use #if, #elif, #else, and #endif to include or exclude code based on whether a symbol is defined. The most common symbols are DEBUG (set automatically for Debug builds) and target framework symbols like NET10_0_OR_GREATER:

static void ConfigureLogging()
{
#if DEBUG
    Console.WriteLine("Debug logging enabled — verbose output active.");
#else
    Console.WriteLine("Release logging — errors only.");
#endif
}

The build system defines the DEBUG symbol when you build in the Debug configuration. You don't need to define it yourself. Target framework symbols like NET10_0_OR_GREATER and NET8_0_OR_GREATER let you write code that adapts to different .NET versions in multi-targeting projects.

You can combine symbols with logical operators: && (and), || (or), and ! (not):

static void ShowPlatformInfo()
{
#if NET10_0_OR_GREATER
    Console.WriteLine("Running on .NET 10 or later.");
#elif NET8_0_OR_GREATER
    Console.WriteLine("Running on .NET 8 or 9.");
#else
    Console.WriteLine("Running on an older .NET version.");
#endif
}

Use #define at the top of a file to define your own symbols. You can also define symbols for the entire project by using the DefineConstants property in your project file.

Warning suppression

Use #pragma warning disable to suppress specific compiler warnings, and #pragma warning restore to re-enable them. Always scope the suppression as narrowly as possible:

    static void ProcessData()
    {
        try
        {
            // Some operation that might fail
            var data = File.ReadAllText("config.json");
            Console.WriteLine($"Config loaded: {data.Length} characters");
        }
#pragma warning disable CS0168 // Variable is declared but never used
        catch (FileNotFoundException ex)
#pragma warning restore CS0168
        {
            // Fall back to defaults — the exception details aren't needed here
            Console.WriteLine("Config file not found, using defaults.");
        }
    }

Tip

Always specify the warning number, such as CS0168, rather than disabling all warnings. This approach keeps the suppression targeted and makes it clear why a warning is being suppressed.

Related content

• C# preprocessor directives (language reference)
• File-based apps