c# - Why does Visual Studio Type a Newly Minted Array as Nullable?

Question

I'm writing a function with a generic type TVal. I wrote this line:

var zeroBased = new TVal[size];

And then in Visual Studio (VS) I used alt+enter to replace var with an explicit type. Here's what I got:

TVal[]? zeroBased = new TVal[size];

I was surprised to find the ? operator, indicating that the type might be nullable. I thought that I'd be safe enough assuming the type is never null when created with new, and could have just done:

TVal[] zeroBased = new TVal[size];

Is there a scenario where instantiating a new array in C# can give you back null?

Note: the code seems to compile fine without the ?, I'm just intrigued by VS's suggestion...

Minimal Verifiable Example

Open Visual Studio, same version as specified below, create a new project, enable nullable types as per the VS Project File Contents below, create a new class, and pop in this function:

public void Test<T>(int size)
{
  var tArr = new T[size];
}

The select the var and hit alt+enter, and choose to replace var with explicit type. If the behaviour is the same as the one I experienced, you'll get:

public void Test<T>(int size)
{
  T[]? tArr = new T[size];
}

Visual Studio Project File Contents

We're using C# 8 for this project and we've enabled Nullables:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <Nullable>enable</Nullable>
    <LangVersion>8.0</LangVersion>
    <WarningsAsErrors>CS8600;CS8602;CS8603</WarningsAsErrors>
    <TargetFramework>netstandard2.0</TargetFramework>
    <OutputType>Library</OutputType>
    <Version>1.0.0.9</Version>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
    <PackageReference Include="System.Dynamic.Runtime" Version="4.3.0" />
  </ItemGroup>

</Project>

Visual Studio Version Info (just bits that seemed important to this Q)

Microsoft Visual Studio Community 2019 Version 16.6.1 VisualStudio.16.Release/16.6.1+30128.74 Microsoft .NET Framework Version 4.7.03062

Installed Version: Community

C# Tools 3.6.0-4.20251.5+910223b64f108fcf039012e0849befb46ace6e66 C# components used in the IDE. Depending on your project type and settings, a different version of the compiler may be used.

Answer 1

I would like to extend an existing answer by adding some links

C# specification proposal says:

nullable implicitly typed local variables

var infers an annotated type for reference types. For instance, in var s = ""; the var is inferred as string?.

It means that var for reference types infers a nullable reference type. This works if nullable context is enabled either using project file or #nullable pragma.

This behavior was discussed in this LDM and implemented in this issue.

This is a reason for making var infer a nullable reference type:

At this point we've seen a large amount of code that requires people spell out the type instead of using var, because code may assign null later.