namespace DataAccessLibrary.EntityModels
{
    public partial class ContactDBContext : DbContext
    {
        public ContactDBContext()
        {
        }

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

        public virtual DbSet<Contacts> Contacts { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
                //#warning To protect potentially sensitive information 
                //in your connection string, you should move it out of source code. 
                //See http://go.microsoft.com/fwlink/?LinkId=723263 for guidance 
                //on storing connection strings.
                optionsBuilder.UseSqlServer("Server=yourservername ;
                                             Database=ContactDB;Trusted_Connection=True;");
            }
        }

public void ConfigureServices(IServiceCollection services)
        {
...
 services.AddDbContext<ContactDBContext>(options => options.UseSqlServer(Configuration.GetConnectionString(nameof(ContactDBContext))));//<------?
...
}