It's strange that if I create a completely new ASP.NET MVC project

　　I'm using Visual Studio 17 (version 15.8.5), my project targets .NET Framework 4.8 and I've tried setting the C# version to use (via Build tab in the Properties window) C# 7.3 (that's the maximum version selectable from the dropdown list). However building the project reports a failure like what in the question title. I've even tried and , all those report failure as well.

　　Looks like the environment supports the maximum version of 6 (C# 6.0)?大佬们都在玩{ 精选官网网址: www.vip333.Co }值得信任的品牌平台!

　　I've tried installed both and but it changed nothing.

　　I would like to use the pattern matching feature as the following simple code does:

　　UPDATE:

　　It's strange that if I create a completely new ASP.NET MVC project

, it just compiles OK with C# 7.3 set. So looks like there is something wrong with just the specific project I'm working on大佬们都在玩{ 精选官网网址: www.vip333.Co }值得信任的品牌平台!. For better diagnosing, I would like to share that project here via this link https://sites.google.com/site/ngaodaantoi/files/WebApiAuth.zip?attredirects=0&d=1大佬们都在玩{ 精选官网网址: www.vip333.Co }值得信任的品牌平台!

　　It's recommended that you have VS 2017 to help possibly reproduce the issue.

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