How can I pass value from child window to parent window by ComboBox (WPF and MVVM)

Question

I have a child window and there is a ComboBox. I want to send value of child window to parent window using WPF and MVVM. Can anyone help me how to do it ??

Answer 1

Don't focus on MVVM for this scenario, MVVM is not designed for passing values from child windows back to parent windows.

Instead, if you have some code in a viewmodel or the code behind of a view that spawns or opens the child window, then that code should be responsible for retrieving the value determined by the child window and propagating it back to the appropriate property (at which point any binding will reflect the value back in the UI - this is where MVVM should be used).

The best thing you can do is make sure the code that opens the child window doesn't go directly accessing a ComboBox on the child window, instead the child window should bind it to a property which is then accessed by the parent's code (preferably via an interface).

For further reading check out Creating an MVVM friendly dialog strategy. This should be your preferred solution, then the parent code simply uses the Dialog Service to show the child window, and the Dialog Service is responsible for aggregating the child window result and making it available back to the calling code in the parent window.

Answer 2

Since you havent given adequate info so, lets assume the child window is a dialog. Now, lets assume the child window is a class Child() with its ViewModel having the object in Child class, say

public ChildViewModel chVM { get; set; } 

and this viewmodel having the property:

public string ComboBoxSelectedValue { get; set; }

Lets have the xaml of the dialog having the combobox as this :-

<ComboBox Name="cbTest" SelectedItem="{Binding ComboBoxSelectedValue}">
<ComboBoxItem>A</ComboBoxItem>
<ComboBoxItem>B</ComboBoxItem>
<ComboBoxItem>C</ComboBoxItem>
</ComboBox>

Now, everytime a value is selected in the combobnox, the property in its viewmodel that is, ComboBoxSelectedValue will be filled with the selected value.

You have to handle the Close event of the dialog on your parent page. Lets move on to the class Parent() that is the parent page:

public partial class Parent : Page
{
    private Child ch;

    public Parent()
    {
        InitializeComponent();

        ch= new Child();
        ch.Closed += ChildClosed;
    }

    public void ChildClosed(object sender, System.EventArgs e)
    {
        //even after closing of child window
        var selectedValue = ch.chVM.ComboBoxSelectedValue;
    }

    public void OpenChild(object sender, System.EventArgs e)
    {
        //Button event to open the child window
        ch.Show();
    }
}

Please respond if this is the one you needed. Or else please feel free to ask for another solution. This can be done without mvvm also, but since you asked for MVVM, so this is the solution.

Answer 3

The best solution I have found for allowing view models to communicate with each other is via a messaging framework, my preference is MVVM light available on nuget.

Your child view model sends a message via the framework, which the parent subscribes to. Not dissimilar to event handlers.

Child...

Messenger.Default.Send<MyMessageClass>(message);

That can go in a combo box selected item binding setter, or part of a command action method.

Parent...

Messenger.Default.Register<MyMessageClass>(this, OnMessage);

The MyMessageClass must extend BaseMessage and should include properties for the data you want to share. The OnMessage method in the parent should accept this class as a parameter and do whatever you need it to in the parent with those values.

Is is better MVVM since it keeps the logic out of code behind or view, but also doesn't create strong couplings. If a view model sends a message that no other view model receives, nothing happens - you also use message objects rather than inspection of view models to share data.

Tutorial on msdn here.