wpf replace dictionary with events

Question

I have a WPF application with two viewmodels(AViewModel,BViewModel). I think that main problem is that Singleton.Instance.Handle.Add("...",(m)=>{...}); cannot be removed from Model because it have High Coupling. And every time when i want to update collection i need to past Singleton.Instance.Handle.Add("...",(m)=>{...}); How to update collection without pasting every time Singleton.Instance.Handle.Add("...",(m)=>{...});

public class AViewModel
{
    AModel A = new AModel();
}

and BViewModel:

public class BViewModel
{
    BModel B = new BModel();
}

AModel:

public class AModel
{
    public ObservableCollection<DataType> AItems { get; set; }
    public AModel()
    {
        AItems = new ObservableCollection<DataType>();
        UpdateAModel();
    }
    public void UpdateAModel()
    {
        // Problem with High Coupling, can not remove into another class
        Singleton.Instance.Handle.Add("MessageB", (m) =>
        {
            Dictionary<string, DataType> d = m.Message.Json.GetFirstArgAs<Dictionary<string, DataType>>();
            foreach (KeyValuePair<string, DataType> item in d)
            {
                // Update AItems
            }
            // Many strings of code
            // Update AItems, problem with add or remove items in ObservableCollection because AItems on UI Thread
        });
    }
}

BModel:

public class BModel
{
    public ObservableCollection<DataType> BItems { get; set; }
    public BModel()
    {
        BItems = new ObservableCollection<DataType>();
        UpdateBModel();
    }
    public void UpdateBModel()
    {
        // Problem with High Coupling, can not remove into another class
        Singleton.Instance.Handle.Add("MessageA", (m) =>
        {
            Dictionary<string, DataType> d = m.Message.Json.GetFirstArgAs<Dictionary<string, DataType>>();
            foreach (KeyValuePair<string,DataType> item in d)
            {
                // Update BItems
            }
            // Many strings of code
            // Update BItems, problem with add or remove items in ObservableCollection because AItems on UI Thread
        });
    }
}

Both models contain ObservableList, which updates with data from web.

public class Singleton
{
    private static Singleton instance;

    public Dictionary<string, Action<MessageEventArgs>> Handle { get; set; }
    private Singleton() 
    {
        Handle = new Dictionary<string, Action<MessageEventArgs>>();
        socket = new Client(UrlSocketServer);
        socket.Message += Message;
    }

    public static Singleton Instance
    {
        get
        {
            if (instance == null)
            {
                instance = new Singleton();
            }
            return instance;
        }
    }
    private void Message(object sender, MessageEventArgs e)
    {
            if (Handle.ContainsKey(e.Message.Event))
            {
                Handle[e.Message.Event](e);
            }
    }
}

Answer 1

You'll get better responses to questions like this if you provide code that compiles independently.

To answer your question, you can improve readability by adding a function:

public void Filter(string key, Action<MessageEventArgs> filter)
{
    Singleton.Instance.Handle.Add(key filter);
}

Secondly, you don't have to use a lambda, you can use regular methods:

private void MessageB(MessageEventArgs m)
{
    Dictionary<string, DataType> d = m.Message.Json.GetFirstArgAs<Dictionary<string, DataType>>();
    foreach (KeyValuePair<string, DataType> item in d)
    {
        // Update AItems
    }
}

So then all you need to do is this:

Filter("MessageB", MessageB);