无法构建.Net core 2.2项目？ 找不到类型或名称空间名称“系统”

Question

I just install the newest .Net core at this moment.

PS C:\temp> dotnet --version
2.2.301

And I created a new console project.

PS C:\temp> dotnet new console

---

Welcome to .NET Core!

Learn more about .NET Core: https://aka.ms/dotnet-docs Use 'dotnet --help' to see available commands or visit: https://aka.ms/dotnet-cli-docs

Telemetry

The .NET Core tools collect usage data in order to help us improve your experience. The data is anonymous and doesn't include command-line arguments. The data is collected by Microsoft and shared with the community. You can opt-out of telemetry by setting the DOTNET_CLI_TELEMETRY_OPTOUT environment variable to '1' or 'true' using your favorite shell.

Read more about .NET Core CLI Tools telemetry: https://aka.ms/dotnet-cli-telemetry

ASP.NET Core

Successfully installed the ASP.NET Core HTTPS Development Certificate. To trust the certificate run 'dotnet dev-certs https --trust' (Windows and macOS only). For establishing trust on other platforms refer to the platform specific documentation. For more information on configuring HTTPS see https://go.microsoft.com/fwlink/?linkid=848054 . Getting ready... The template "Console Application" was created successfully.

Processing post-creation actions... Running 'dotnet restore' on C:\\temp\\temp.csproj... Restore completed in 190.58 ms for C:\\temp\\temp.csproj.

Restore succeeded.

---

However, dotnet run got the following error?

PS C:\temp> dotnet run
C:\Users\wangyi\AppData\Local\Temp\1\.NETCoreApp,Version=v2.2.AssemblyAttributes.cs(4,20): error CS0400: The type or namespace name 'System' could not be found in the global namespace (are you missing an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(10,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(11,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(12,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(13,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(14,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(15,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(16,12): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
C:\Users\wangyi\AppData\Local\Temp\1\.NETCoreApp,Version=v2.2.AssemblyAttributes.cs(4,71): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
C:\Users\wangyi\AppData\Local\Temp\1\.NETCoreApp,Version=v2.2.AssemblyAttributes.cs(4,99): error CS0246: The type or namespace name 'FrameworkDisplayName' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
C:\Users\wangyi\AppData\Local\Temp\1\.NETCoreApp,Version=v2.2.AssemblyAttributes.cs(4,122): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(10,55): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(11,61): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(12,59): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(13,68): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(14,55): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(15,53): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(16,55): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
Program.cs(1,7): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
C:\Users\wangyi\AppData\Local\Temp\1\.NETCoreApp,Version=v2.2.AssemblyAttributes.cs(2,7): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
C:\Users\wangyi\AppData\Local\Temp\1\.NETCoreApp,Version=v2.2.AssemblyAttributes.cs(3,7): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(7,7): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
obj\Debug\netcoreapp2.2\temp.AssemblyInfo.cs(8,7): error CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) [C:\temp\temp.csproj]
Program.cs(5,11): error CS0518: Predefined type 'System.Object' is not defined or imported [C:\temp\temp.csproj]
Program.cs(7,26): error CS0518: Predefined type 'System.String' is not defined or imported [C:\temp\temp.csproj]
Program.cs(7,16): error CS0518: Predefined type 'System.Void' is not defined or imported [C:\temp\temp.csproj]

The build failed. Please fix the build errors and run again.

Running dotnet restore didn't get any error.

PS C:\temp> dotnet restore
Restore completed in 41.15 ms for C:\temp\temp.csproj.

I tried "deleting the C:\\Program Files\\dotnet\\sdk\\NuGetFallbackFolder" ( https://github.com/dotnet/core/issues/1006 ) but it still didn't resolve the issue.

However, in visual studio, the project compiles and runs after I switch the .net core version of the project to 2.1 .

Answer 1

Try to run:

dotnet restore

Then it should work, or at least give you information about the reason why the dependencies cannot be resolved.

Answer 2

In the command prompt you can write: dotnet --version and you will see your current version. I recommend you download the core framework from here instead of installing through the visual studio installer. Installing .net core from the site fixed my problems.

Answer 3

The problem is gone today. I tried to install the newest .Net core SDK and it still didn't resolve the problem.

Then I launched VS 2019 and created a new .Net core project in VS. And it compiles. And I go back to the powershell and found out dotnet run works now.