private void getCategoryList() { var t = new Task(() => { }); t.Start(); t.ContinueWith(task => { Connector databaseConnectCategory = new MYSQLDatabaseConnection(); if (databaseConnectCategory.establish() == true) { List <DatabaseColumn> CategoryList = databaseConnectCategory.GetAllCategoryNameWithId(); foreach (DatabaseColumn databseColumn in CategoryList) { cmbCategoryName.Items.Add(databseColumn.category_name);//add company name into combo box TempCategoryList.Add(new DatabaseColumn() { category_name = databseColumn.category_name, category_id = databseColumn.category_id, } ); } } else { MessageBox.Show("connection error"); } }, System.Threading.CancellationToken.None, TaskContinuationOptions.None, TaskScheduler.FromCurrentSynchronizationContext()); }