/langversion 的选项“4”无效；必须是 ISO-1、ISO-2、3 或 Default SystemFrameWorkV3
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version
Edit the csproj file
You can set the language version in your .csproj file. Add an element like the following:
XML
<PropertyGroup>
<LangVersion>latest</LangVersion>
</PropertyGroup>
The value latest uses the latest minor version of the C# language. Valid values are:
| Value | Meaning |
|---|---|
| default | The compiler accepts all valid language syntax from the latest major version that it can support. |
| ISO-1 | The compiler accepts only syntax that is included in ISO/IEC 23270:2003 C# (1.0/1.2) |
| ISO-2 | The compiler accepts only syntax that is included in ISO/IEC 23270:2006 C# (2.0) |
| 3 | The compiler accepts only syntax that is included in C# 3.0 or lower. |
| 4 | The compiler accepts only syntax that is included in C# 4.0 or lower. |
| 5 | The compiler accepts only syntax that is included in C# 5.0 or lower. |
| 6 | The compiler accepts only syntax that is included in C# 6.0 or lower. |
| 7 | The compiler accepts only syntax that is included in C# 7.0 or lower. |
| 7.1 | The compiler accepts only syntax that is included in C# 7.1 or lower. |
| 7.2 | The compiler accepts only syntax that is included in C# 7.2 or lower. |
| 7.3 | The compiler accepts only syntax that is included in C# 7.3 or lower. |
| latest | The compiler accepts all valid language syntax that it can support. |
The special strings default and latest resolve to the latest major (C# 7.0) and minor (C# 7.3) language versions installed on the build machine, respectively.
