清除NuGet缓存后,LanguageVersion不包含C#9的定义

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在中单击“清除所有NuGet缓存”按钮后

Visual Studio > Tools Options > NuGet Package Manager > General

属性Microsoft。代码分析。CSharp。语言版本。源代码中的CSharp9

CSharpParseOptions.Default.WithLanguageVersion(Microsoft.CodeAnalysis.CSharp.LanguageVersion.CSharp9);

引发编译错误

错误CS0117“LanguageVersion”不包含的定义 'CSharp9'

所有NuGet软件包都是最新的。如何解决此问题?程序集信息不包含CSharp9成员:

#region Assembly Microsoft.CodeAnalysis.CSharp, Version=3.6.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35
// location unknown
// Decompiled with ICSharpCode.Decompiler 6.1.0.5902
#endregion

namespace Microsoft.CodeAnalysis.CSharp
{
    //
    // Summary:
    //     Specifies the language version.
    public enum LanguageVersion
    {
        //
        // Summary:
        //     C# language version 1
        CSharp1 = 1,
        //
        // Summary:
        //     C# language version 2
        CSharp2 = 2,
        //
        // Summary:
        //     C# language version 3
        //
        // Remarks:
        //     Features: LINQ.
        CSharp3 = 3,
        //
        // Summary:
        //     C# language version 4
        //
        // Remarks:
        //     Features: dynamic.
        CSharp4 = 4,
        //
        // Summary:
        //     C# language version 5
        //
        // Remarks:
        //     Features: async, caller info attributes.
        CSharp5 = 5,
        //
        // Summary:
        //     C# language version 6
        //
        // Remarks:
        //     Features:
        //     • Using of a static class
        //     • Exception filters
        //     • Await in catch/finally blocks
        //     • Auto-property initializers
        //     • Expression-bodied methods and properties
        //     • Null-propagating operator ?.
        //     • String interpolation
        //     • nameof operator
        //     • Dictionary initializer
        CSharp6 = 6,
        //
        // Summary:
        //     C# language version 7.0
        //
        // Remarks:
        //     Features:
        //     • Out variables
        //     • Pattern-matching
        //     • Tuples
        //     • Deconstruction
        //     • Discards
        //     • Local functions
        //     • Digit separators
        //     • Ref returns and locals
        //     • Generalized async return types
        //     • More expression-bodied members
        //     • Throw expressions
        CSharp7 = 7,
        //
        // Summary:
        //     C# language version 7.1
        //
        // Remarks:
        //     Features:
        //     • Async Main
        //     • Default literal
        //     • Inferred tuple element names
        //     • Pattern-matching with generics
        CSharp7_1 = 701,
        //
        // Summary:
        //     C# language version 7.2
        //
        // Remarks:
        //     Features:
        //     • Ref readonly
        //     • Ref and readonly structs
        //     • Ref extensions
        //     • Conditional ref operator
        //     • Private protected
        //     • Digit separators after base specifier
        //     • Non-trailing named arguments
        CSharp7_2 = 702,
        //
        // Summary:
        //     C# language version 7.3
        CSharp7_3 = 703,
        //
        // Summary:
        //     C# language version 8.0
        CSharp8 = 800,
        //
        // Summary:
        //     The latest major supported version.
        LatestMajor = 2147483645,
        //
        // Summary:
        //     Preview of the next language version.
        Preview = 2147483646,
        //
        // Summary:
        //     The latest supported version of the language.
        Latest = int.MaxValue,
        //
        // Summary:
        //     The default language version, which is the latest supported version.
        Default = 0
    }
}

微软代码分析。CSharp.dll文件不是从任何anyprojet直接引用的。

NuGet窗口不显示任何名称中包含roslyn的包。