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    <p>An easy way to achieve this would be to use IoC. When you create your child view model, pass a reference of your main view model to your child viewmodel , and hold it as a private read only variable. You now access all of your main VM publics . </p> <p>An alternative solution would ppossibly be to use the Mediator pattern.</p> <p>Knocked up a simple App to demonstrate IoC solution.</p> <p>App.xaml.cs</p> <pre><code>public partial class App : Application { protected override void OnStartup(StartupEventArgs e) { base.OnStartup(e); var window = new MainWindow() {DataContext = new MainWindowViewModel() }; window.Show(); } } </code></pre> <p>MainWindowViewModel.cs</p> <pre><code>public class MainWindowViewModel : ViewModelBase { private string _statusMessage; public string StatusMessage { get { return _statusMessage; } set { _statusMessage = value; this.OnPropertyChanged("StatusMessage"); } } public ICommand OpenChildCommand { get; private set; } public MainWindowViewModel() { this.StatusMessage = "No status"; this.OpenChildCommand = new DelegateCommand((o) =&gt; this.OpenChild()); } private void OpenChild() { var window = new ChildWindow {DataContext = new ChildWindowViewModel(this)}; window.Show(); } } </code></pre> <p>ChildWindowViewModel.cs</p> <pre><code>public class ChildWindowViewModel : ViewModelBase { private readonly MainWindowViewModel _mainvm; public ChildWindowViewModel(MainWindowViewModel mainvm) { _mainvm = mainvm; this.UpdateStatusCommand = new DelegateCommand((o) =&gt; this.UpdateStatus()); } public ICommand UpdateStatusCommand { get; private set; } private void UpdateStatus() { this._mainvm.StatusMessage = "New Status"; } } </code></pre> <p>ViewModelBase.cs</p> <pre><code>public abstract class ViewModelBase : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; protected void OnPropertyChanged(string propertyName) { this.OnPropertyChanged(new PropertyChangedEventArgs(propertyName)); } protected virtual void OnPropertyChanged(PropertyChangedEventArgs e) { var handler = this.PropertyChanged; if (handler != null) { handler(this, e); } } } </code></pre> <p>DelegateCommand.cs</p> <pre><code>public class DelegateCommand : ICommand { /// &lt;summary&gt; /// Action to be performed when this command is executed /// &lt;/summary&gt; private Action&lt;object&gt; executionAction; /// &lt;summary&gt; /// Predicate to determine if the command is valid for execution /// &lt;/summary&gt; private Predicate&lt;object&gt; canExecutePredicate; /// &lt;summary&gt; /// Initializes a new instance of the DelegateCommand class. /// The command will always be valid for execution. /// &lt;/summary&gt; /// &lt;param name="execute"&gt;The delegate to call on execution&lt;/param&gt; public DelegateCommand(Action&lt;object&gt; execute) : this(execute, null) { } /// &lt;summary&gt; /// Initializes a new instance of the DelegateCommand class. /// &lt;/summary&gt; /// &lt;param name="execute"&gt;The delegate to call on execution&lt;/param&gt; /// &lt;param name="canExecute"&gt;The predicate to determine if command is valid for execution&lt;/param&gt; public DelegateCommand(Action&lt;object&gt; execute, Predicate&lt;object&gt; canExecute) { if (execute == null) { throw new ArgumentNullException("execute"); } this.executionAction = execute; this.canExecutePredicate = canExecute; } /// &lt;summary&gt; /// Raised when CanExecute is changed /// &lt;/summary&gt; public event EventHandler CanExecuteChanged { add { CommandManager.RequerySuggested += value; } remove { CommandManager.RequerySuggested -= value; } } /// &lt;summary&gt; /// Executes the delegate backing this DelegateCommand /// &lt;/summary&gt; /// &lt;param name="parameter"&gt;parameter to pass to predicate&lt;/param&gt; /// &lt;returns&gt;True if command is valid for execution&lt;/returns&gt; public bool CanExecute(object parameter) { return this.canExecutePredicate == null ? true : this.canExecutePredicate(parameter); } /// &lt;summary&gt; /// Executes the delegate backing this DelegateCommand /// &lt;/summary&gt; /// &lt;param name="parameter"&gt;parameter to pass to delegate&lt;/param&gt; /// &lt;exception cref="InvalidOperationException"&gt;Thrown if CanExecute returns false&lt;/exception&gt; public void Execute(object parameter) { if (!this.CanExecute(parameter)) { throw new InvalidOperationException("The command is not valid for execution, check the CanExecute method before attempting to execute."); } this.executionAction(parameter); } } </code></pre> <p>MainWindow.xaml</p> <pre><code>&lt;Window x:Class="WpfApplication2.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="350" Width="525"&gt; &lt;StackPanel&gt; &lt;TextBlock Text="{Binding StatusMessage, Mode=OneWay}" /&gt; &lt;Button HorizontalAlignment="Center" VerticalAlignment="Center" Content="Open Child Window" Command="{Binding Path=OpenChildCommand}"/&gt; &lt;/StackPanel&gt; </code></pre> <p></p> <p>ChildWindow.xaml</p> <pre><code>&lt;Window x:Class="WpfApplication2.ChildWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="ChildWindow" Height="300" Width="300"&gt; &lt;Grid&gt; &lt;Button HorizontalAlignment="Center" VerticalAlignment="Center" Content="UpdateStatus" Command="{Binding Path=UpdateStatusCommand}" /&gt; &lt;/Grid&gt; </code></pre> <p></p> <p>Image before clicking on update status</p> <p><img src="https://i.stack.imgur.com/rrjB6.png" alt="Before"></p> <p>Image after clicking on updatestatus</p> <p><img src="https://i.stack.imgur.com/ZQ5JG.png" alt="enter image description here"></p>
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    1. COI did pass the main view model while initiating ChildVM and set the MainVM property some value. But it did nor reflected over the page.. no idea why.
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    2. COAre you implementing INPC on the mainvm and calling propertychanged on the mainvm property?
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    3. COThis is wat im doing in my MainMV public string StatusMessage { get { return statusMsg; } set { statusMsg = value; OnPropertyChanged("StatusMessage"); } } Then while initializing the child vm im doing ChildVM(mainVM vm) { _main = vm; _main.StatusMessage = "new status"; Is it the right way?
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