Which C# version .NET Core uses?

.NET Core 2.0 references Roslyn 2.3, which corresponds to Visual Studio 2017 version 15.3 and supports C# 7.1.

The C# what's new version history page gives a list of all versions plus their associated Visual Studio and .NET core version:

• C# 7.3 Visual Studio 2017 version 15.7, and in the .NET Core 2.1 SDK 2.1.300 RC1
• C# 7.2 Visual Studio 2017 version 15.5, and in the .NET Core 2.0 SDK.
• C# 7.1 Visual Studio 2017 version 15.3, and in the .NET Core 2.0 SDK.
• C# 7.0 Visual Studio 2017 and .NET Core 1.0 and later

C# 8.0 is still in preview at this time (3-Jul-2019).

You can also see your SDK version with this command:

dotnet --info

Sample output:

.NET Core SDK (reflecting any global.json):
 Version:   2.1.300
 Commit:    adab45bf0c

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  10.13
 OS Platform: Darwin
 RID:         osx.10.13-x64
 Base Path:   /usr/local/share/dotnet/sdk/2.1.300/

Host (useful for support):
  Version: 2.1.0
  Commit:  caa7b7e2ba

.NET Core SDKs installed:
  2.1.300 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

From Microsoft .net core what's new page:

NET Core 2.0 supports C# 7.1, which adds a number of new features, including:

• The Main method, the application entry point, can be marked with the async keyword.
• Inferred tuple names.
• Default expressions.

You can also review check the C# Language versioning page

The compiler determines a default based on these rules:

.NET Core 2.x C# 7.3