如何模拟多重继承并使用反射来优化此代码？

时间：2009-06-22 14:12:37

标签： c# wpf inheritance abstract-class multiple-inheritance

我有一个WPF应用程序，其中 PageItems 是模型对象。

我的主ViewModel有一个 PageItemViewModels 的ObservableCollection，每个都来自其匹配的PageItem模型对象。

每个 PageItemViewModel 都从抽象类 BaseViewModel 继承，以获得INotifyPropertyChanged功能。

每个 PageItemViewModel 还会实现 IPageItemViewModel ，以确保它具有所需的属性。

我最终会有大约50页，所以我想消除任何不必要的代码：

  • 已解决（见下文）：有没有办法让PageItemViewModel类能够继承IdCode和Title ，所以我不必在每个类中实现它们？我不能把它们放在BaseViewModel中，因为其他ViewModel继承它不需要这些属性，我不能把它们放在IPageItemViewModel中，因为它只是一个接口。我知道我需要多重继承，因为C＃不支持
  • 已解决（见下文）：有没有办法摆脱开关声明，例如：以某种方式使用反射代替？

以下是独立控制台应用，它演示了我在 WPF 应用中的代码：

using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<PageItem> pageItems = PageItems.GetAll();
            List<ViewModelBase> pageItemViewModels = new List<ViewModelBase>();

            foreach (PageItem pageItem in pageItems)
            {
                switch (pageItem.IdCode)
                {
                    case "manageCustomers":
                        pageItemViewModels.Add(new PageItemManageCustomersViewModel(pageItem));
                        break;
                    case "manageEmployees":
                        pageItemViewModels.Add(new PageItemManageEmployeesViewModel(pageItem));
                        break;
                    default:
                        break;
                }
            }
        }
    }

    public class PageItemManageCustomersViewModel : ViewModelBase, IPageItemViewModel
    {
        public string IdCode { get; set; }
        public string Title { get; set; }

        public PageItemManageCustomersViewModel(PageItem pageItem)
        {

        }
    }

    public class PageItemManageEmployeesViewModel : ViewModelBase, IPageItemViewModel
    {
        public string IdCode { get; set; }
        public string Title { get; set; }

        public PageItemManageEmployeesViewModel(PageItem pageItem)
        {

        }
    }

    public interface IPageItemViewModel
    {
        //these are the properties which every PageItemViewModel needs
        string IdCode { get; set; }
        string Title { get; set; }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItems
    {
        public static List<PageItem> GetAll()
        {
            List<PageItem> pageItems = new List<PageItem>();
            pageItems.Add(new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers"});
            pageItems.Add(new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees"});
            return pageItems;
        }
    }

}

重构：接口更改为抽象类

using System;
using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<PageItem> pageItems = PageItems.GetAll();
            List<ViewModelPageItemBase> pageItemViewModels = new List<ViewModelPageItemBase>();

            foreach (PageItem pageItem in pageItems)
            {
                switch (pageItem.IdCode)
                {
                    case "manageCustomers":
                        pageItemViewModels.Add(new PageItemManageCustomersViewModel(pageItem));
                        break;
                    case "manageEmployees":
                        pageItemViewModels.Add(new PageItemManageEmployeesViewModel(pageItem));
                        break;
                    default:
                        break;
                }
            }

            foreach (ViewModelPageItemBase pageItemViewModel in pageItemViewModels)
            {
                System.Console.WriteLine("{0}:{1}", pageItemViewModel.IdCode, pageItemViewModel.Title);
            }
            Console.ReadLine();
        }
    }

    public class PageItemManageCustomersViewModel : ViewModelPageItemBase
    {
        public PageItemManageCustomersViewModel(PageItem pageItem)
        {
            IdCode = pageItem.IdCode;
            Title = pageItem.Title;
        }
    }

    public class PageItemManageEmployeesViewModel : ViewModelPageItemBase
    {
        public PageItemManageEmployeesViewModel(PageItem pageItem)
        {
            IdCode = pageItem.IdCode;
            Title = pageItem.Title;
        }
    }

    public abstract class ViewModelPageItemBase : ViewModelBase
    {
        //these are the properties which every PageItemViewModel needs
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItems
    {
        public static List<PageItem> GetAll()
        {
            List<PageItem> pageItems = new List<PageItem>();
            pageItems.Add(new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers"});
            pageItems.Add(new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees"});
            return pageItems;
        }
    }

}

消除Switch语句的答案：

谢谢Jab：

string assemblyName = System.Reflection.Assembly.GetExecutingAssembly().GetName().Name;
string viewModelName = assemblyName + ".ViewModels.PageItem" + StringHelpers.ForcePascalNotation(pageItem.IdCode) + "ViewModel";
var type = Type.GetType(viewModelName);
var viewModel = Activator.CreateInstance(type, pageItem) as ViewModelBase;
AllPageViewModels.Add(viewModel);

5 个答案:

答案 0 :(得分：2)

您是否可以创建一个继承BaseViewModel的类来实现这两个属性 - 您需要的PageItemViewModel类可以从中继承。

答案 1 :(得分：1)

根据Paddy的建议，我刚刚创建了一个额外的抽象类，即PageViewModelBase，并定义了这些自动道具：

using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<PageItem> pageItems = PageItems.GetAll();
            List<ViewModelBase> pageItemViewModels = new List<ViewModelBase>();

            foreach (PageItem pageItem in pageItems)
            {
                switch (pageItem.IdCode)
                {
                    case "manageCustomers":
                        pageItemViewModels.Add(new PageItemManageCustomersViewModel(pageItem));
                        break;
                    case "manageEmployees":
                        pageItemViewModels.Add(new PageItemManageEmployeesViewModel(pageItem));
                        break;
                    default:
                        break;
                }
            }
        }
    }

    public class PageItemManageCustomersViewModel : PageViewModelBase
    {
        public PageItemManageCustomersViewModel(PageItem pageItem)
        {

        }
    }

    public class PageItemManageEmployeesViewModel : PageViewModelBase
    {
        public PageItemManageEmployeesViewModel(PageItem pageItem)
        {


        }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public abstract class PageViewModelBase : ViewModelBase
    {
        //these are the properties which every PageItemViewModel needs
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItems
    {
        public static List<PageItem> GetAll()
        {
            List<PageItem> pageItems = new List<PageItem>();
            pageItems.Add(new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers"});
            pageItems.Add(new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees"});
            return pageItems;
        }
    }

}

答案 2 :(得分：1)

为什么不能在基础PageItem类中放置一个GetViewModel()虚拟方法来返回相应的视图模型？

   foreach (PageItem pageItem in pageItems)
   {
       pageItemViewModels.Add(pageItem.GetViewModel());
   }

立即看起来像代码味道的东西是“id”属性的用法 - 这通常可以用多态替换。所以你要用上面的代码替换switch语句。

修改

如果您的PageItem类对您的视图模型一无所知，那么就无法以这种方式实现它。基本上，你需要一个你已经拥有的工厂（在某种程度上）。

我通常有一个关系列表（PageItem到ViewModel），在你的情况下是Dictionary<String, Type>。然后，您可以在初始化期间填写此列表，并在以后实例化适当的视图模型。

要使用反射来构建此列表，您至少需要以编程方式了解视图模型支持哪个页面项。为此，您可以使用自定义属性来装饰您的类，例如：

public class SupportsPageItemAttribute : Attribute
{
    private readonly string _id;
    public string ID
    {
        get { return _id;}
    }

    public SupportsPageItemAttribute(string id)
    {
        _id = id;
    }
}

然后使用该属性来定义模型可以接受的PageItem：

[SupportsPageItemAttribute("manageCustomers")
public class PageItemManageCustomersViewModel
{
   // ...
}

然后，使用反射来获取所有实现IPageItemViewModel的类，并检查它们的属性以获取PageItem id字符串。

例如（没有太多的错误检查）：

Dictionary<String, Type> modelsById = new Dictionary<String, Type>();
String viewModelInterface = typeof(IPageItemViewModel).FullName;

// get the assembly
Assembly assembly = Assembly.GetAssembly(typeof(IPageItemViewModel));

// iterate through all types
foreach (Type viewModel in assembly.GetTypes())
{
    // get classes which implement IPageItemViewModel
    if (viewModel.GetInterface(viewModelInterface) != null)
    {
        // get the attribute we're interested in
        foreach (Attribute att in Attribute.GetCustomAttributes(viewModel))
        {
            if (att is SupportsPageItemAttribute)
            {
                // get the page item id
                String id = (att as SupportsPageItemAttribute).ID;

                // add to dictionary
                modelsById.Add(id, viewModel);
            }
        }
    }
}

另一方面，您可以考虑使用各种反转控制框架，而不是自己进行讨厌的反射工作。

答案 3 :(得分：1)

一个不太漂亮但有效的解决方案是使用约定来摆脱switch语句。这假设您可以更改IdCodes或至少修改案例以匹配ViewModel。

    var type = Type.GetType("PageItem" + pageItem.IdCode + "ViewModel");
    var viewModel = Activator.CreateInstance(type) as ViewModelBase;
    pageItemViewModels.Add(viewModel);

请注意，您应该在此添加错误检查，这里有几点失败。然而，它比维持不断增长的转换声明更好。

答案 4 :(得分：1)

一种可能的解决方案是在代码中反转PageItem和PageItemViewModel之间的关系。现在，您正在基于PageItemViewModel生成PageItem，但是如果先创建PageItemViewModel然后在每个PageItemViewModel的构造函数中创建，那么您创建了适当的PageItem？这消除了对switch的需求并使事情更清晰，因为现在您的视图模型负责模型，而不是负责视图模型的模型。

基于您当前代码的示例：

using System;
using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<ViewModelPageItemBase> pageItemViewModels = PageItemViewModels.GetAll();

            // No switch needed anymore. Each PageItem's view-model contains its PageItem
            // which is exposed as property of the view-model.
            foreach (ViewModelPageItemBase pageItemViewModel in pageItemViewModels)
            {
                System.Console.WriteLine("{0}:{1}", pageItemViewModel.PageItem.IdCode, pageItemViewModel.PageItem.Title);
            }
            Console.ReadLine();
        }
    }

    public class PageItemManageCustomersViewModel : ViewModelPageItemBase
    {
        public PageItemManageCustomersViewModel()
        {
            PageItem = new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers" };
        }
    }

    public class PageItemManageEmployeesViewModel : ViewModelPageItemBase
    {
        public PageItemManageEmployeesViewModel()
        {
            PageItem = new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees" };
        }
    }

    public abstract class ViewModelPageItemBase : ViewModelBase
    {
        //The PageItem associated with this view-model
        public PageItem PageItem { get; protected set; }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    // Replaces PageItems class
    public class PageItemViewModels
    {
        // Return a list of PageItemViewModel's instead of PageItem's.
        // Each PageItemViewModel knows how to build it's corresponding PageItem object.
        public static List<PageItemViewModelBase> GetAll()
        {
            List<PageItemViewModelBase> pageItemViewModels = new List<PageItemViewModelBase>();
            pageItemViewModels.Add(new PageItemManageCustomersViewModel());
            pageItemViewModels.Add(new PageItemManageEmployeesViewModel());
            return pageItemViewModels;
        }
    }
}
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