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==Non-standard features ==
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The <code>#pragma</code> directive is defined by standard languages, but with little or no requirements for syntax after its name so that compilers are free to define subsequent syntax and associated behavior. For instance, a pragma is often used to allow suppression of error messages, manage heap and stack debugging and so on.
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Some [[Unix]] preprocessors provided an [[assertion (computing)|assertion]] feature {{endash}} which has little similarity to standard library assertions.<ref>[https://gcc.gnu.org/onlinedocs/cpp/Obsolete-Features.html GCC Obsolete features]</ref>
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GCC provides <code>#include_next</code> for chaining headers of the same name.<ref>{{Cite web|url=https://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html|title = Wrapper Headers (The C Preprocessor)}}</ref>
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Unlike C and C++, Objective-C includes an <code>#import</code> directive that is like <code>#include</code> but results in a file being included only once {{endash}} eliminating the need for include guards and [[pragma once|<code>#pragma once</code>]].
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The null directive, which consists only of the <code>#</code> character, alone on a single line, is a non-standard directive in Microsoft Visual C++. It has no effect.<ref>{{Cite web|title=Null directive|url=https://learn.microsoft.com/en-us/cpp/preprocessor/null-directive|publisher=learn.microsoft.com}}</ref>
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The <code>#nullable</code> directive in C# is used to enable and disable nullable reference types. To enable them, use <code>#nullable enable</code>, and <code>#nullable disable</code> to disable them.
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This directive does not exist in C/C++.
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The <code>#region</code> and <code>#endregion</code> directives in C# are used to expand/collapse sections of code in IDEs, and has no effect on actual compilation of the program. It is primarily used for code organisation and readability.
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[[C++/CLI]] has the <code>#using</code> directive, which is used to import metadata into a program from a Microsoft Intermediate Language file (such as a {{mono|.dll}} file).<ref>{{Cite web|title=#using directive (C++/CLI)|url=https://learn.microsoft.com/en-us/cpp/preprocessor/hash-using-directive-cpp|publisher=learn.microsoft.com|date=29 June 2022}}</ref>
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