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Check that link https://www.reddit.com/r/VisualStudio/comments/b8vbj8/help_netf461_project_opened_in_vs2019_all_files/
Right click on "References" in the Solution Explorer
Select "Manage NuGet Packages"
Under the "Installed" tab search fro "Microsoft.Net.Compilers"
Select "Microsoft.Net.Compilers"
On the side window of "Microsoft.Net.Compilers" click the update button
VS2015 is compatible with .NET Compiler 1.x.x
VS2017 is compatible with .NET Compiler 2.x.x
VS2019 is compatible with .NET Compiler 3.x.x
So, just update Microsoft.Net.Compilers according to you vs's version