VS Code CSC : error CS1617: Invalid option '7.3' for /langversion

Question

I downloaded VS 2017 15.7, .NET Core 2.1.2 and Blazor to try it out. It wasn't working in VS 2017 properly and thought I would try through the dotnet cli and VS Code. Instead I was met with the following error:

CSC : error CS1617: Invalid option '7.3' for /langversion.

In VS, it will run the application show "Loading..." and then nothing. Not sure if the two issues are related

dotnet --info returns the following:

.NET Command Line Tools (2.1.300-preview1-008174)  Product Information:  Version:            2.1.300-preview1-008174 Commit SHA-1 hash:  b8df89a54f  Runtime Environment:  OS Name:     Windows  OS Version:  10.0.17666  OS Platform: Windows  RID:         win10-x64  Base Path:   C:\Program Files\dotnet\sdk\2.1.300-preview1-008174\  Microsoft .NET Core Shared Framework Host    Version  : 2.1.0-preview1-26216-03   Build    : f2c3216183d20416568a4bbf5bb7d153e826f153 

Answer 1

Just had this and a Clean, Rebuild fixed the issue for me.

Answer 2

On VS 2019, I went to project files, changed the target framework to 4.7.2 from 4.6.2, the project build successfully. Then revert back to 4.6.2 and it still builds successfully