Table per Type in Entity Framework Core 2.0

You would have to change your Model to look like this, note that you can't use inheritance using this approach:

public class Company
{
   public int CompanyId { get; set; }
   //...
}

public class Company
{
   public int CompanyId { get; set; }
   public string Name { get; set; }
   //...
}

public class HeadOffice
{
   [ForeignKey(nameof(Company))]
   public int CompanyId { get; set; }
   public Company Company { get; set; }
   // Add Properties here
}

public class BranchOffice
{
   [ForeignKey(nameof(Company))]
   public int CompanyId { get; set; }
   public Company Company { get; set; }
   // Add Properties here
}

Your DbContext:

public class YourContext : DbContext
{
  public DbSet<Company> Companys { get; set; }
  public DbSet<HeadOffice> HeadOffices { get; set; }
  public DbSet<BranchOffice> BranchOffices { get; set; }

  public YourContext(DbContextOptions<YourContext> options)
    : base(options)
  {
  }
}

You could then use EF Core Migrations. The command would look somewhat like this:

dotnet ef migrations add Initial_TPT_Migration -p ./../../ModelProject.csproj -s ./../../ModelProject.csproj -c YourContext -o ./TptModel/CodeFirst/Migrations

It generats a Class Initial_TPT_Migration that contains methods to generate your database.

Usage

To query you would need to map Company Properties to the fieldnames. If you combine this with the Repository Pattern (link), it could actually be as convenient as the default approach in EF Core currently is to use.

YourContext ctx = ...

// Fetch all BranchOffices
var branchOffices = ctx.BranchOffices
          .Select(c => new BranchOffice()
                  {
                    CompanyId = c.CompanyId,
                    Name = c.Company.Name,
                  })
          .ToList();

You can find more informations about this approach here.