DependencyResolver.SetResolver(new UnityDependencyResolver(container));

public partial class frm_FirmaSecimi : XtraForm
{
    private IFirmaService _firmaService;

    public frm_FirmaSecimi()
    {
        InitializeComponent();
        _firmaService = UnityConfig.Container.Resolve<IFirmaService>();
    }
}

public partial class frm_FirmaSecimi : XtraForm
{
    private IFirmaService _firmaService;

    public frm_FirmaSecimi(IFirmaService firmaService)
    {
        InitializeComponent();
        _firmaService = firmaService;
    }
}