How to enable Nullable Reference Types feature of C# 8.0 for the whole project

In Visual Studio 16.2 (from preview 1) the property name is changed to Nullable, which is simpler and aligns with the command line argument.

Add the following properties to your .csproj file.

<PropertyGroup>
  <Nullable>enable</Nullable>
  <LangVersion>8.0</LangVersion>
</PropertyGroup>

If you're targeting netcoreapp3.0 or later, you don't need to specify a LangVersion to enable nullable reference types.

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For older Visual Studio versions:

• From 16.0 preview 2 to 16.1, set NullableContextOptions to enable.
• In 16.0 preview 1, set NullableReferenceTypes to true.

Note that this setting is changed between VS 2019 preview 1 and preview 2. With preview 2 or 3, you need this in your .csproj:

<PropertyGroup>
  <LangVersion>8.0</LangVersion>
  <NullableContextOptions>enable</NullableContextOptions>
</PropertyGroup>

The <NullableReferenceTypes> mentioned in the earlier answer (which, when I originally wrote this answer on 4th Feb 2019, had been marked as the accepted answer) was correct at the time that answer was written, but it is no longer recognized.

In addition to @DrewNoakes accepted answer, note that the nullable property can be set for all projects at once by adding a file called Directory.Build.props in the folder that contains your .sln file.

Just define your Directory.Build.props file like this:

<Project>

  <PropertyGroup>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

You will need to restart Visual Studio for this to take effect.

More about Directory.Build.props.