<ComboBox x:Name="cBx_SubCategories"  ItemsSource="{Binding ListSubCategories}" DisplayMemberPath="nameSubCategory" />

Thread thread = new Thread(this.StartFill_DDL_SubCategories);
            thread.Start(idUser);

private List<SubCategory> listSubCategories;
        public List<SubCategory> ListSubCategories          
        {
            get
            {
                return listSubCategories;
            }
            set
            {
                listSubCategories = value;
            }
        }

public void StartFill_DDL_SubCategories(object idUser)
        {            
                this.listSubCategories = salesmens.GetStartSubCategories((int)idUser);                            
        }