Entity Framework Core Customize Scaffolding

You can use DbContextWriter & EntityTypeWriter to customize scaffold output.

In newer versions of entity core writers renamed:

• DBContextWriter ==>> CSharpDbContextGenerator
• EntityTypeWriter ==>> CSharpEntityTypeGenerator

Write some custom type writer, you can override everything and you will get your own code generator:

//HERE YOU CAN CHANGE THE WAY TYPES ARE GENERATED AND YOU CAN ADD INTERFACE OR BASE CLASS AS PARENT.
public class CustomEntitiyTypeWriter : EntityTypeWriter
{
    public CustomEntitiyTypeWriter([NotNull] CSharpUtilities cSharpUtilities)
        : base(cSharpUtilities)
    { }

    // Write Code returns generated code for class and you can raplec it with your base class
    public override string WriteCode([NotNull] EntityConfiguration entityConfiguration)
    {
        var classStr = base.WriteCode(entityConfiguration);

        var defaultStr = "public partial class " + entityConfiguration.EntityType.Name;
        var baseStr = "public partial class " + entityConfiguration.EntityType.Name + " : EntityBase";

        classStr = classStr.Replace(defaultStr, baseStr);

        return classStr;
    }      
}

declare it in setup:

public static void ConfigureDesignTimeServices(IServiceCollection services)
           => services.AddSingleton<EntityTypeWriter, CustomEntitiyTypeWriter>();

and then scaffold db, you can do the same for DBContext with CustomDBContextWriter.

@Tornike Choladze's excellent answer led me in the right direction, but in the latest versions of .Net Core (2.0 >) this has to be done a little differently it seems, with regard to the setup.

The custom entity type generator:

class MyEntityTypeGenerator : CSharpEntityTypeGenerator
{
    public MyEntityTypeGenerator(ICSharpUtilities cSharpUtilities) : base(cSharpUtilities) { }

    public override string WriteCode(IEntityType entityType, string @namespace, bool useDataAnnotations)
    {
        string code = base.WriteCode(entityType, @namespace, useDataAnnotations);

        var oldString = "public partial class " + entityType.Name;
        var newString = "public partial class " + entityType.Name + " : EntityBase";

        return code.Replace(oldString, newString);
    }
}

And the setup, which consists of a class in the same assembly and implementing IDesignTimeServices:

public class MyDesignTimeServices : IDesignTimeServices
{
    public void ConfigureDesignTimeServices(IServiceCollection serviceCollection)
    {
        serviceCollection.AddSingleton<ICSharpEntityTypeGenerator, MyEntityTypeGenerator>();            
    }
}

In case you want to modify entity names (and file and class names) here's something that might help:

Based on Chris Peacock's answer (and comments) you can build two classes to modify the names of entities and files (this works in Core 2.2).

public class CustomEFUtilities : CSharpUtilities
{
    public override string Uniquifier(
        string proposedIdentifier, ICollection<string> existingIdentifiers)
    {
        var finalIdentifier = base.Uniquifier(proposedIdentifier, existingIdentifiers);
        
        // your changes here
        if (finalIdentifier.StartsWith("tl"))
        {
            finalIdentifier = finalIdentifier.Substring(2);
        }
        
        return finalIdentifier;
    }
}

And similarly:

public class CustomEFDesignTimeServices : IDesignTimeServices
{
    public void ConfigureDesignTimeServices(IServiceCollection serviceCollection)
    {
        serviceCollection.AddSingleton<ICSharpUtilities, CustomEFUtilities>();
    }
}

Edit (EF Core 3.1)

A breaking change was introduced (https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-3.0/breaking-changes#microsoftentityframeworkcoredesign-is-now-a-developmentdependency-package) so you need to modify your project file:

If you need to reference this package to override EF Core's design-time behavior, then you can update PackageReference item metadata in your project.

<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="3.0.0">
  <PrivateAssets>all</PrivateAssets>
  <!-- Remove IncludeAssets to allow compiling against the assembly -->
  <!--<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>-->
</PackageReference>