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C# Compiler Options for language feature rules

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The following options control how the compiler interprets language features. The new MSBuild syntax is shown in Bold. The older csc.exe syntax is shown in code style.

  • CheckForOverflowUnderflow / -checked: Generate overflow checks.
  • AllowUnsafeBlocks / -unsafe: Allow 'unsafe' code.
  • DefineConstants / -define: Define conditional compilation symbol(s).
  • LangVersion / -langversion: Specify language version such as default (latest major version), or latest (latest version, including minor versions).
  • Nullable / -nullable: Enable nullable context, or nullable warnings.

CheckForOverflowUnderflow

The CheckForOverflowUnderflow option controls the default overflow-checking context that defines the program behavior if integer arithmetic overflows.

<CheckForOverflowUnderflow>true</CheckForOverflowUnderflow>

When CheckForOverflowUnderflow is true, the default context is a checked context and overflow checking is enabled; otherwise, the default context is an unchecked context. The default value for this option is false, that is, overflow checking is disabled.

You can also explicitly control the overflow-checking context for the parts of your code by using the checked and unchecked statements.

For information about how the overflow-checking context affects operations and what operations are affected, see the article about checked and unchecked statements.

AllowUnsafeBlocks

The AllowUnsafeBlocks compiler option allows code that uses the unsafe keyword to compile. The default value for this option is false, meaning unsafe code isn't allowed.

<AllowUnsafeBlocks>true</AllowUnsafeBlocks>

For more information about unsafe code, see Unsafe Code and Pointers.

DefineConstants

The DefineConstants option defines symbols in all source code files of your program.

<DefineConstants>name;name2</DefineConstants>

This option specifies the names of one or more symbols that you want to define. The DefineConstants option has the same effect as the #define preprocessor directive except that the compiler option is in effect for all files in the project. A symbol remains defined in a source file until an #undef directive in the source file removes the definition. When you use the -define option, an #undef directive in one file has no effect on other source code files in the project. You can use symbols created by this option with #if, #else, #elif, and #endif to compile source files conditionally. The C# compiler itself defines no symbols or macros that you can use in your source code; all symbol definitions must be user-defined.

Note

The C# #define directive does not allow a symbol to be given a value, as in languages such as C++. For example, #define cannot be used to create a macro or to define a constant. If you need to define a constant, use an enum variable. If you want to create a C++ style macro, consider alternatives such as generics. Since macros are notoriously error-prone, C# disallows their use but provides safer alternatives.

LangVersion

The default language version for the C# compiler depends on the target framework for your application and the version of the SDK or Visual Studio installed. Those rules are defined in C# language versioning.

The LangVersion option causes the compiler to accept only syntax that is included in the specified C# language specification, for example:

<LangVersion>9.0</LangVersion>

The following values are valid:

Value Meaning
preview The compiler accepts all valid language syntax from the latest preview version.
latest The compiler accepts syntax from the latest released version of the compiler (including minor version).
latestMajor
or default		 The compiler accepts syntax from the latest released major version of the compiler.
12.0 The compiler accepts only syntax that is included in C# 12 or lower.
11.0 The compiler accepts only syntax that is included in C# 11 or lower.
10.0 The compiler accepts only syntax that is included in C# 10 or lower.
9.0 The compiler accepts only syntax that is included in C# 9 or lower.
8.0 The compiler accepts only syntax that is included in C# 8.0 or lower.
7.3 The compiler accepts only syntax that is included in C# 7.3 or lower.
7.2 The compiler accepts only syntax that is included in C# 7.2 or lower.
7.1 The compiler accepts only syntax that is included in C# 7.1 or lower.
7 The compiler accepts only syntax that is included in C# 7.0 or lower.
6 The compiler accepts only syntax that is included in C# 6.0 or lower.
5 The compiler accepts only syntax that is included in C# 5.0 or lower.
4 The compiler accepts only syntax that is included in C# 4.0 or lower.
3 The compiler accepts only syntax that is included in C# 3.0 or lower.
ISO-2
or 2		 The compiler accepts only syntax that is included in ISO/IEC 23270:2006 C# (2.0).
ISO-1
or 1		 The compiler accepts only syntax that is included in ISO/IEC 23270:2003 C# (1.0/1.2).

Important

The latest value is generally not recommended. With latest, the compiler enables the latest features, even if those features depend on updates not included in the configured target framework.

Considerations

  • To ensure that your project uses the default compiler version recommended for your target framework, don't use the LangVersion option. You can update the target framework to access newer language features.

  • Specifying LangVersion with the default value is different from omitting the LangVersion option. Specifying default uses the latest version of the language that the compiler supports, without taking into account the target framework. For example, building a project that targets .NET 6 from Visual Studio version 17.6 uses C# 10 if LangVersion isn't specified, but uses C# 11 if LangVersion is set to default.

  • Metadata referenced by your C# application isn't subject to the LangVersion compiler option.

  • Because each version of the C# compiler contains extensions to the language specification, LangVersion doesn't give you the equivalent functionality of an earlier version of the compiler.

  • While C# version updates generally coincide with major .NET releases, the new syntax and features aren't necessarily tied to that specific framework version. Each specific feature has its own minimum .NET API or common language runtime requirements that may allow it to run on downlevel frameworks by including NuGet packages or other libraries.

  • Regardless of which LangVersion setting you use, use the current version of the common language runtime to create your .exe or .dll. One exception is friend assemblies and ModuleAssemblyName, which work under -langversion:ISO-1.

For other ways to specify the C# language version, see C# language versioning.

For information about how to set this compiler option programmatically, see LanguageVersion.

C# language specification

Version Link Description
C# 8.0 and later download PDF C# Language Specification Version 7: .NET Foundation
C# 7.3 download PDF Standard ECMA-334 7th Edition
C# 6.0 download PDF Standard ECMA-334 6th Edition
C# 5.0 Download PDF Standard ECMA-334 5th Edition
C# 3.0 Download DOC C# Language Specification Version 3.0: Microsoft Corporation
C# 2.0 Download PDF Standard ECMA-334 4th Edition
C# 1.2 Download DOC Standard ECMA-334 2nd Edition
C# 1.0 Download DOC Standard ECMA-334 1st Edition

Minimum SDK version needed to support all language features

The following table lists the minimum versions of the SDK with the C# compiler that supports the corresponding language version:

C# version Minimum SDK version
C# 12 Microsoft Visual Studio/Build Tools 2022 version 17.8, or .NET 8 SDK
C# 11 Microsoft Visual Studio/Build Tools 2022 version 17.4, or .NET 7 SDK
C# 10 Microsoft Visual Studio/Build Tools 2022, or .NET 6 SDK
C# 9.0 Microsoft Visual Studio/Build Tools 2019 version 16.8, or .NET 5 SDK
C# 8.0 Microsoft Visual Studio/Build Tools 2019, version 16.3, or .NET Core 3.0 SDK
C# 7.3 Microsoft Visual Studio/Build Tools 2017, version 15.7
C# 7.2 Microsoft Visual Studio/Build Tools 2017, version 15.5
C# 7.1 Microsoft Visual Studio/Build Tools 2017, version 15.3
C# 7.0 Microsoft Visual Studio/Build Tools 2017
C# 6 Microsoft Visual Studio/Build Tools 2015
C# 5 Microsoft Visual Studio/Build Tools 2012 or bundled .NET Framework 4.5 compiler
C# 4 Microsoft Visual Studio/Build Tools 2010 or bundled .NET Framework 4.0 compiler
C# 3 Microsoft Visual Studio/Build Tools 2008 or bundled .NET Framework 3.5 compiler
C# 2 Microsoft Visual Studio/Build Tools 2005 or bundled .NET Framework 2.0 compiler
C# 1.0/1.2 Microsoft Visual Studio/Build Tools .NET 2002 or bundled .NET Framework 1.0 compiler

Nullable

The Nullable option lets you specify the nullable context. It can be set in the project's configuration using the <Nullable> tag:

<Nullable>enable</Nullable>

The argument must be one of enable, disable, warnings, or annotations. The enable argument enables the nullable context. Specifying disable will disable the nullable context. When you specify the warnings argument, the nullable warning context is enabled. When you specify the annotations argument, the nullable annotation context is enabled. The values are described and explained in the article on Nullable contexts. You can learn more about the tasks involved in enabling nullable reference types in an existing codebase in our article on nullable migration strategies.

Note

When there's no value set, the default value disable is applied, however the .NET 6 templates are by default provided with the Nullable value set to enable.

Flow analysis is used to infer the nullability of variables within executable code. The inferred nullability of a variable is independent of the variable's declared nullability. Method calls are analyzed even when they're conditionally omitted. For instance, Debug.Assert in release mode.

Invocation of methods annotated with the following attributes will also affect flow analysis:

Important

The global nullable context does not apply for generated code files. Regardless of this setting, the nullable context is disabled for any source file marked as generated. There are four ways a file is marked as generated:

  1. In the .editorconfig, specify generated_code = true in a section that applies to that file.
  2. Put <auto-generated> or <auto-generated/> in a comment at the top of the file. It can be on any line in that comment, but the comment block must be the first element in the file.
  3. Start the file name with TemporaryGeneratedFile_
  4. End the file name with .designer.cs, .generated.cs, .g.cs, or .g.i.cs.

Generators can opt-in using the #nullable preprocessor directive.