The type or namespace name 'ApplicationUser' couldn't be found

I've included Identity into my .NET Core3.1 project, when I try to build the project, it warns me with the following error:

The type or namespace name 'ApplicationUser' couldn't be found (are you missing a using directive or an assembly reference?)

You'll have to create the ApplicationUser class yourself. The default user is IdentityUser. ApplicationUser inherits from IdentityUser:

public class ApplicationUser : IdentityUser { }

But if you are not extending ApplicationUser then you can also use IdentityUser. So instead of this:

services.AddIdentity<ApplicationUser, IdentityRole>()

you can use this:

services.AddIdentity<IdentityUser, IdentityRole>()

And replace ApplicationUser throughout the rest of the project with IdentityUser.

参考：

https://stackoverflow.com/questions/52015990/asp-net-core-2-1-identity-couldnt-find-applicationuser

https://docs.microsoft.com/en-us/aspnet/core/security/authentication/customize-identity-model?view=aspnetcore-3.1#custom-user-data