问 找不到指定的框架'Microsoft.AspNetCore.App'，版本'2.1.0‘

我尝试将我的web应用程序从.net Core2.0升级到.net Core2.1，我做到了：

1)安装Visual Studio专业版2017预览版15.8.0预览版1.1

2)从这里安装了.net core 2.1.3 RC1 SDK： https://www.microsoft.com/net/download/dotnet-core/sdk-2.1.300-rc1

3)将我所有的aspnetcore nuget包更新到最新版本。

在做完这两件事之后，当我开始我的项目时，我会看到这个屏幕：

超文本传输协议错误502.5 -进程失败此问题的常见原因：

应用程序进程无法启动已启动的应用程序进程但随后停止已启动的应用程序进程但未能在配置的端口上侦听

故障排除步骤：

检查系统事件日志中的错误消息启用日志记录应用程序进程的标准输出消息将调试器附加到应用程序进程并检查

欲了解更多信息，请访问： https://go.microsoft.com/fwlink/?LinkID=808681 。

在我的ASP.NET核心Web服务器的输出窗口中，我得到了这个：

The specified framework 'Microsoft.AspNetCore.App', version '2.1.0' was not found.
  - Check application dependencies and target a framework version installed at:
      C:\Program Files\dotnet\
  - Installing .NET Core prerequisites might help resolve this problem:
      http://go.microsoft.com/fwlink/?LinkID=798306&clcid=0x409
  - The .NET Core framework and SDK can be installed from:
      https://aka.ms/dotnet-download
  - The following versions are installed:
      2.1.0-preview2-final at [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
      2.1.0-rc1-final at [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
It was not possible to find any compatible framework version
The specified framework 'Microsoft.AspNetCore.App', version '2.1.0' was not found.
  - Check application dependencies and target a framework version installed at:
      C:\Program Files\dotnet\
  - Installing .NET Core prerequisites might help resolve this problem:
      http://go.microsoft.com/fwlink/?LinkID=798306&clcid=0x409
  - The .NET Core framework and SDK can be installed from:
      https://aka.ms/dotnet-download
  - The following versions are installed:
      2.1.0-preview2-final at [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
      2.1.0-rc1-final at [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]