private async void button2_Click(object sender, EventArgs e) { this.comboBoxListDiscipline.SelectedIndexChanged += new EventHandler(comboboxListDiscipline_SelectedIndexChanged); await Task.Factory.StartNew(() => { InvokeUI(() => { var _listGroup = WorkWithDB.GetListGroupForDiscipline(DisciplineName).AsParallel(); if (WorkWithDB.GetListGroupForDiscipline(DisciplineName) != null) { AddItemToComboBox(comboBoxListGroups, _listGroup); } else { MessageBox.Show("Таких групп нет!"); } _listGroup.ToList().Clear(); }); }); }