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A source code only package which allows you to use .NET Core 3.0's new nullable attributes in older target frameworks like .NET Standard 2.0 or the "old" .NET Framework.
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Nullable Nuget

Use .NET Core 3.0's new nullable attributes in older target frameworks.

🏃 Quickstart   |   📚 Guides   |   📦 NuGet

With the release of C# 8.0, support for nullable reference types has been added to the language. Futhermore, .NET Core 3.0 added new nullable attributes like the AllowNullAttribute which are sometimes required to exactly declare when and how null is allowed in specific code sections.

Unfortunately, these attributes are not available in older target frameworks like .NET Standard 2.0 which makes annotating existing code harder. Luckily, this problem can be solved by re-declaring the attributes as internal classes - the C# compiler will still use them for generating warnings, even though the target framework doesn't support these attributes by itself.

This repository hosts the code for the "Nullable" NuGet Package which adds a single C# source code file redefining all of the new nullable attributes to projects which reference the package.

This file is added at compile time and gets built into the referencing project. This means that the resulting project does not have an explicit dependency on the Nullable package, because the attributes are not distributed as a library.

Futhermore, the file only gets added to the project, if the nullable attributes are not already supported by the target framework. The images below show an example - in the library which targets .NET Standard 2.0, the attributes have been added during the compilation. That is not the case for the library targeting .NET Standard 2.1, because the attributes are available through the .NET BCL there. This allows you to easily multi-target your projects without having to change a single line of code.

.NET Standard 2.0 .NET Standard 2.1
.NET Standard 2.0 .NET Standard 2.1


Nullabe is currently compatible with the following target frameworks:

  • .NET Standard >= 1.0
  • .NET Framework >= 2.0

Please have a look at the guides for additional information on how to install the package for your target framework.


⚠️ Important:
You must use C# 8.0 with the Nullable package - otherwise, your project won't compile.

Currently, you can not use the package with a packages.config file. See this issue for details.
You may use this guide to see how you can upgrade from a packages.config to package references.

The steps below will only work with the new SDK .csproj style! Other installation guides can be found here.

  1. Reference the package
    Add the package to your project, for example via:

    Install-Package Nullable
    dotnet add package Nullable
  2. Ensure that the package has been added as a development dependency
    Open your .csproj file and ensure that the new package reference looks similar to this:

    <PackageReference Include="Nullable" Version="<YOUR_VERSION>">
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>

    This is especially important for libraries that are published to NuGet, because without this, the library will have an explicit dependency on the Nullable package.

  3. Build the project
    Ensure that the project compiles. If a build error occurs, you will most likely have to update the C# language version.

Afterwards, you can immediately start using the attributes.

Compiler Constants

The included C# file makes use of some compiler constants that can be used to enable or disable certain features.


If the NULLABLE_ATTRIBUTES_DISABLE constant is defined, the attributes are excluded from the build. This can be used to conditionally exclude the attributes from the build if they are not required.

In most cases, this should not be required, because the package automatically excludes the attributes from target frameworks that already support these attributes.


Because the attributes are added as source code, they may appear in code coverage reports.

By defining the NULLABLE_ATTRIBUTES_EXCLUDE_FROM_CODE_COVERAGE constant, the ExcludeFromCodeCoverage attribute gets added to the nullable attribute classes, thus preventing them from appearing in the reports.

⚠️ Important:
This is disabled by default, because certain target frameworks like .NET Standard 1.7 don't support the ExcludeFromCodeCoverage attribute. Ensure that this attribute is supported by your target framework before defining this constant.


Because the package consists of source files, building works differently than a normal .NET project. In essence, no build has to be made at all. Instead, the *.cs files are renamed to *.cs.pp (because otherwise, Visual Studio's solution explorer would display the files in a project which references the package) and then packaged into a NuGet package via a .nuspec file.

The build.ps1 file in the repository's root automates this process with a few extra benefits. Run this file for manually creating the package.


I don't expect this package to require many changes, but if something is not working for you or if you think that the source file should change, feel free to create an issue or Pull Request. I will be happy to discuss and potentially integrate your ideas!


See the LICENSE file for details.

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