如何在HomeViewModel中调用MainViewModel的TestEvent方法？Prism使用报错

我有HomeViewModel和MainViewModel，想要在HomeViewModel中调用MainViewModel的TestEvent方法，尝试使用Prism框架的IEventAggregator实现，但在MainViewModel构造函数中传入IEventAggregator参数时，持续出现以下错误：

Exception thrown: 'System.MissingMethodException' in System.Xaml.dll
Exception thrown: 'System.Xaml.XamlObjectWriterException' in System.Xaml.dll
Exception thrown: 'System.NullReferenceException' in System.Private.CoreLib.dll
Object reference not set to an instance of an object.

同时，若在HomeViewModel构造函数中传入IEventAggregator也会出现类似错误。

MainViewModel代码 #

using Learning.Core;
using Learning.EventRegister;
using Learning.MVVM.Model;
using Prism.Events;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;

namespace Learning.MVVM.ViewModel
{
    public class MainViewModel: ObservableObject
    {
        public RelayCommand HomeViewConmmand { get; set; }
        public RelayCommand DiscoveryViewConmmand { get; set; }
        public RelayCommand TestCommand { get; set; }

        public HomeViewModel HomeVM { get; set; }
        public DiscoveryViewModel DiscoveryVM { get; set; }
        public RoomViewModel RoomVM { get; set; }

        private object _currentView;
        private object _rightView;

        IEventAggregator _eventAggregator;

        public object CurrentView
        {
            get { return _currentView; }
            set { 
                _currentView = value;
                OnPropertyChanged();
            }
        }

        public object RightView
        {
            get { return _rightView; }
            set
            {
                _rightView = value;
                OnPropertyChanged();
            }
        }

        public MainViewModel(IEventAggregator ea) {
            HomeVM = new HomeViewModel();
            DiscoveryVM = new DiscoveryViewModel();
            RoomVM = new RoomViewModel();
            CurrentView = HomeVM;
            RightView = RoomVM;

            HomeViewConmmand = new RelayCommand(o =>
            {
                CurrentView = HomeVM;
            });

            DiscoveryViewConmmand = new RelayCommand(o =>
            {
                CurrentView = DiscoveryVM;
            });

            TestCommand = new RelayCommand(o =>
            {
                MessageBox.Show("Clicked");
            });
        }

        public void TestEvent(string message)
        {
            MessageBox.Show(message);
        }
    }
}

HomeViewModel代码 #

using Learning.Core;
using Learning.EventRegister;
using Learning.MVVM.Model;
using Prism.Events;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;

namespace Learning.MVVM.ViewModel
{
    public class HomeViewModel
    {
        // Data
        public List<Room> rooms { get; }

        // Command
        private RelayCommand openRoomCommand;

        //IEventAggregator 
        IEventAggregator _eventAggregator;

        public RelayCommand NotifyCommand { get; set; }

        public HomeViewModel()
        {
            openRoomCommand = new RelayCommand(o =>
            {
                Room cur_room = (Room)o;
                cur_room.IsActive = true;
            });

            rooms = new List<Room>();
            Room room1 = new Room
            {
                Name = "1",
                IsActive = false,
            };
            Room room2 = new Room
            {
                Name = "2",
                IsActive = false,
            };
            Room room3 = new Room
            {
                Name = "3",
                IsActive = false,
            };
            Room room4 = new Room
            {
                Name = "4",
                IsActive = false,
            };

            rooms.Add(room1);
            rooms.Add(room2);
            rooms.Add(room3);
            rooms.Add(room4);
        }

        public void SendEvent()
        {
            //_eventAggregator.GetEvent<CommandSendEvent>().Publish("Something happening");
        }

        public RelayCommand OpenRoomCommand { get => openRoomCommand; }
    }
}

1. 解决构造函数注入错误 #

报错核心原因：

• 手动new ViewModel实例（如HomeVM = new HomeViewModel()），未通过Prism依赖注入容器创建，导致IEventAggregator无法自动注入。
• 若XAML中直接实例化MainViewModel，会尝试调用无参构造函数，但你只定义了带IEventAggregator参数的构造函数，因此抛出MissingMethodException。

修复步骤：

• 确保项目使用Prism依赖注入容器（如Unity、DryIoc），Prism默认已注册IEventAggregator。
• 不要手动new ViewModel，改为通过容器解析：

  public MainViewModel(IEventAggregator ea, HomeViewModel homeVM, DiscoveryViewModel discoveryVM, RoomViewModel roomVM) 
  {
      _eventAggregator = ea;
      HomeVM = homeVM;
      DiscoveryVM = discoveryVM;
      RoomVM = roomVM;
      // 其他初始化代码...
  }
  

• 或启用Prism的ViewModelLocator，让View自动关联ViewModel，无需手动实例化。

2. 用Prism EventAggregator实现跨ViewModel通信 #

步骤1：定义事件类 #

创建继承自PubSubEvent<string>的事件类，用于传递消息：

using Prism.Events;

namespace Learning.EventRegister
{
    public class CommandSendEvent : PubSubEvent<string>
    {
    }
}

步骤2：在MainViewModel中订阅事件 #

在MainViewModel构造函数中订阅事件，关联到TestEvent方法：

public MainViewModel(IEventAggregator ea, HomeViewModel homeVM, DiscoveryViewModel discoveryVM, RoomViewModel roomVM) 
{
    _eventAggregator = ea;
    // 订阅事件，事件发布时调用TestEvent
    _eventAggregator.GetEvent<CommandSendEvent>().Subscribe(TestEvent);
    
    HomeVM = homeVM;
    DiscoveryVM = discoveryVM;
    RoomVM = roomVM;
    // 其他初始化代码...
}

步骤3：在HomeViewModel中注入IEventAggregator并发布事件 #

修改HomeViewModel构造函数，注入IEventAggregator并完善SendEvent方法：

public class HomeViewModel
{
    // ... 其他成员 ...
    
    IEventAggregator _eventAggregator;

    // 修改构造函数，注入IEventAggregator
    public HomeViewModel(IEventAggregator eventAggregator)
    {
        _eventAggregator = eventAggregator;
        
        // 其他初始化代码...
        
        // 给NotifyCommand赋值，触发SendEvent
        NotifyCommand = new RelayCommand(o => SendEvent());
    }

    public void SendEvent()
    {
        _eventAggregator.GetEvent<CommandSendEvent>().Publish("Something happening");
    }

    // ... 其他成员 ...
}

步骤4：确保ViewModel通过容器解析 #

在Prism容器注册代码中（如App.xaml.cs的RegisterTypes方法），注册所有ViewModel：

protected override void RegisterTypes(IContainerRegistry containerRegistry)
{
    containerRegistry.Register<HomeViewModel>();
    containerRegistry.Register<DiscoveryViewModel>();
    containerRegistry.Register<RoomViewModel>();
    containerRegistry.Register<MainViewModel>();
}

容器创建MainViewModel和HomeViewModel时，会自动注入同一个IEventAggregator实例，HomeViewModel发布事件后，MainViewModel的TestEvent方法会被调用。

内容的提问来源于stack exchange，提问作者Văn Hào Trương