/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:

XMLCopy

<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.