WPF ComboBox选中项绑定失效的原因

       <ComboBox
            DisplayMemberPath="FileName"
            ItemsSource="{Binding DbConfigCollection, Mode=TwoWay}"
            SelectedItem="{Binding TaskOptions.TargetDatabase, Mode=TwoWay}" />

        public CreateTaskViewModel()
        {
            TaskOptions = new TaskOptions();
            this.schedulerFactory = schedulerFactory;

            var configs = DbConfigHelper.GetDbConfigs();
            if (configs != null) DbConfigCollection = new ObservableCollection<DbConfig>(configs);

            CronExpressions = CronExpressionHelper.GetCronExpressions();
        }

SelectedItem选中项的对象必须在ItemsSource数据源中，是同一对象。TaskOptions和DbConfigCollection在初始化页面时创建，而TaskOptions.TargetDatabase指向的并不是DbConfigCollection中的对象，需要重新绑定。

           if (selectTask.FirstOrDefault() != null)
            {
                viewModel.TaskOptions = selectTask.FirstOrDefault();
                viewModel.TaskOptions.Database = viewModel.DbConfigCollection.Where(c => c.FileName == selectTask.FirstOrDefault().Database.FileName).FirstOrDefault();
                viewModel.TaskOptions.TargetDatabase = viewModel.DbConfigCollection.Where(c => c.FileName == selectTask.FirstOrDefault().TargetDatabase.FileName).FirstOrDefault();
                viewModel.IsEdit = true;
            }