使用级联组合框进行急切加载

Question

我有一个实体框架应用程序，其中客户和项目之间存在一对多关系。在级联组合框中访问数据。我最初使用延迟加载并且一切正常。由于我现在必须序列化我的数据，我需要以另一种方式加载我的数据。我想使用急切加载，但我不确定如何在我的情况下实现它。在 Projects 属性中，我尝试使用以下内容进行访问：

 var a = context.Customers.Include("Project");
            projects = (ObservableCollection<Project>)a;

但出现转换类型错误。有人对我访问产品数据有什么建议吗？谢谢。

public class ViewModel:INotifyPropertyChanged
{
    RevisionModelContainer context = new RevisionModelContainer();

    public ViewModel()
    {
        Customers = new ObservableCollection<Customer>();            
    }
 private ObservableCollection<Customer> customers;
    public ObservableCollection<Customer> Customers
    {
        get 
        {         
            return customers;            
        }            
        set
        {
            customers = value;
            OnPropertyChanged("Customers");               
        }
    }

    private ObservableCollection<Project> projects;
    public ObservableCollection<Project> Projects 
    {
        get
        {
            var a = context.Customers.Include("Project");
            projects = (ObservableCollection<Project>)a;
            return projects;             
        }
        set
        {
            projects = value;
            OnPropertyChanged("Projects");
        }
    }

 private Customer selectedCustomer;
    public Customer SelectedCustomer
    {
        get { return selectedCustomer; }
        set
        {
            try
            {
                selectedCustomer = value;
                OnPropertyChanged("SelectedCustomer");
                this.Projects = selectedCustomer.Projects;
                OnPropertyChanged("Projects");
            }
            catch (Exception ex) 
            {
                Debug.WriteLine("Seleceted Customer: " + ex.Message);
            }
        }
    }

    private Project selectedProject;
    public Project SelectedProject
    {
        get { return selectedProject; }
        set
        {
            try
            {
                selectedProject = value;
                OnPropertyChanged("SelectedProject");
                this.Programs = selectedProject.Programs;
                OnPropertyChanged("Programs");
            }
            catch (Exception ex)
            {
                Debug.WriteLine("Selected Project: " + ex.Message);
            }
        }
    }

Answer 1

您的“项目”和“客户”属性应分别为IEnumerable<Project> 和IEnumerable<Customer> 类型。您不需要将任何内容强制转换为 ObservableCollection。唯一需要使用 ObservableCollections 的时候是视图需要对集合的 content 变化做出反应。在您的情况下，视图只需要知道 Projects 属性 value 更改为不同的项目列表。

这就是我的意思：

public class ViewModel : INotifyPropertyChanged
{
    RevisionModelContainer context = new RevisionModelContainer();

    public ViewModel() 
    {
        Customers = context.Customers.Include("Project");
    }

    public IEnumerable<Customer> Customers { get; private set; }

    public IEnumerable<Project> Projects { get; private set; }

    private Customer selectedCustomer;
    public Customer SelectedCustomer
    {
        get 
        { 
            return selectedCustomer; 
        }
        set
        {
            selectedCustomer = value;
            OnPropertyChanged("SelectedCustomer");

            // reset the projects.
            this.SelectedProject = null;
            this.Projects = selectedCustomer.Projects;
            OnPropertyChanged("Projects");
        }
    }

    private Project selectedProject;
    public Project SelectedProject
    {
        get 
        { 
            return selectedProject; 
        }
        set
        {
            selectedProject = value;
            OnPropertyChanged("SelectedProject");
        }
    }
}

根据我对您的用例的了解，这应该可行。如果您在视图模型中从客户、项目或程序集合中动态添加或删除项，您只需要可观察的集合，如下所示：Projects.Add(new Project());。

如果您所做的只是分配像 Projects = SelectedCustomer.Projects 这样的属性，则无需使用可观察集合，您只需为更新的属性引发 propertychanged 事件。