How to call functions in a main view model from other view models?

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The delegate method used in this and the linked answer can be used in any parent-child relationship and in either direction. That includes from a child view model to a parent view model, a Window code behind to the code behind of a child Window, or even pure data relationships without any UI involved. You can find out more about using delegate objects from the Delegates (C# Programming Guide) page on MSDN.

I just answered a similar question to this earlier today. If you take a look at the Passing parameters between viewmodels post, you’ll see that the answer involves using delegate objects. You can simply replace these delegates (from the answer) with your method(s) and it will work in the same way.

Please let me know if you have any questions.

UPDATE >>>

Yes, sorry I completely forgot you wanted to call methods instead… I’ve been working on too many posts tonight. So still using the example from the other post, just call your method in the ParameterViewModel_OnParameterChange handler:

public void ParameterViewModel_OnParameterChange(string parameter)
{
    // Call your method here
}

Think of the delegate as being your path back to the parent view model… it’s like raising an event called ReadyForYouToCallMethodNow. In fact, you don’t even need to have an input parameter. You could define your delegate like this:

public delegate void ReadyForUpdate();

public ReadyForUpdate OnReadyForUpdate { get; set; }

Then in the parent view model (after attaching the handler like in the other example):

public void ChildViewModel_OnReadyForUpdate()
{
    // Call your method here
    UpdateDisplay();
}

As you have multiple child view models, you could define the delegate in another class that they both have access to. Let me know if you have any more questions.

UPDATE 2 >>>

After reading your last comment again, I’ve just thought of a much simpler method that might achieve what you want… at least, if I understand you correctly. It is possible for you to Bind directly from your child views to your parent view model. For instance, this would allow you to Bind a Button.Command property in a child view to an ICommand property in your parent view model:

In TreeViewView:

<Button Content="Click Me" Command="{Binding DataContext.ParentCommand, 
RelativeSource={RelativeSource AncestorType={x:Type MainWindow}}}" />

This of course assumes that an instance of the parent view model in question is set as the DataContext of the MainWindow.

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