添加
后运行$ dotnet restore
<ItemGroup>
<PackageReference Include="EntityFramework" Version="6.2.0"/>
</ItemGroup>`
到DBCreation.csproj
（无论是manualy还是NuGet）没有任何事情发生，只是完成了
Restoring packages for /home/denis/Documents/projects/dotNet/DBCreation/DBCreation.csproj...
且Entity中的using System.Data.Entity;仍标记为
"The type or namespace name 'Entity' does not exist in the namespace 'System.Data' (are you missing an assembly reference?) [DBCreation]"
尝试运行时（只有Hello world！使用此功能）：
$dotnet run
The build failed. Please fix the build errors and run again.
但是
$ dotnet build
Microsoft (R) Build Engine version 15.6.84.34536 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.
Restoring packages for
/home/denis/Documents/projects/dotNet/DBCreation/DBCreation.csproj...
完成restore，但没有显示错误。
当.csproj列表中没有EntityFramework时，恢复完成如
Restoring packages for /home/denis/Documents/projects/dotNet/DBCreation/DBCreation.csproj...
Restore completed in 751.55 ms for /home/denis/Documents/projects/dotNet/DBCreation/DBCreation.csproj.
并且工作正确。
我在Debian 9上使用Visual Studio Code。
$ dotnet --info
.NET Command Line Tools (2.1.105)
Product Information:
Version: 2.1.105
Commit SHA-1 hash: 141cc8d976
Runtime Environment:
OS Name: debian
OS Version: 9
OS Platform: Linux
RID: debian.9-x64
Base Path: /usr/share/dotnet/sdk/2.1.105/
Microsoft .NET Core Shared Framework Host
Version : 2.0.7
Build : 2d61d0b043915bc948ebf98836fefe9ba942be11
如何将Entity Framework添加到项目中，还原有什么问题以及我的错误在哪里？