dotnet ef not found in .NET Core 3

问题

I'm following the docs in order to create an initial migration. When I execute dotnet, I get the help section, meaning that the PATH works properly.

Then I try to execute the command below from the docs in console window:

dotnet ef migrations add InitialCreate  

I get the following error:

Could not execute because the specified command or file was not found.  
Possible reasons for this include:  
  * You misspelled a built-in dotnet command.  
  * You intended to execute a .NET Core program, but dotnet-ef does not exist.  
  * You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.

• I excluded the first item since I copied the command.
• I excluded the second item because the package Microsoft.EntityFrameworkCore.SqlServer is installed.
• I excluded the third item because I get the help section when invoking dotnet.

I'm googling the issue but since the version is new, there's not much to go on and/or it's drowning in similar issues from earlier versions.

I tried to forcibly install Microsoft.EntityFrameworkCore just in case it needs to be explicitly added. I ran into the error message telling me that the latest version to pick from is 2.2.6 and a downgrade is a no-go. I'm not sure how to install the version compatible with the SQL-package I have on my system already (and even less certain if that's right approach to kill this issue).

Detected package downgrade: Microsoft.EntityFrameworkCore from 3.0.0-preview6.19304.10 to 2.2.6. Reference the package directly from the project to select a different version.
Web ->
Microsoft.EntityFrameworkCore.SqlServer 3.0.0-preview6.19304.10 ->
Microsoft.EntityFrameworkCore.Relational 3.0.0-preview6.19304.10 ->
Microsoft.EntityFrameworkCore (>= 3.0.0-preview6.19304.10)
Web -> Microsoft.EntityFrameworkCore (>= 2.2.6)

回答1:

See the announcement for ASP.NET Core 3 Preview 4, which explains that this tool is no longer built-in and requires an explicit install:

The dotnet ef tool is no longer part of the .NET Core SDK

This change allows us to ship dotnet ef as a regular .NET CLI tool that can be installed as either a global or local tool. For example, to be able to manage migrations or scaffold a DbContext, install dotnet ef as a global tool typing the following command:

dotnet tool install --global dotnet-ef --version 3.0.0

The reason for the change is explained in the docs:

Why

This change allows us to distribute and update dotnet ef as a regular .NET CLI tool on NuGet, consistent with the fact that the EF Core 3.0 is also always distributed as a NuGet package.

In addition, you might need to add the following NuGet packages to your project:

• Microsoft.EntityFrameworkCore.SqlServer
• Microsoft.EntityFrameworkCore.Design
• Microsoft.EntityFrameworkCore.Tools

来源：https://stackoverflow.com/questions/57066856/dotnet-ef-not-found-in-net-core-3