how to pass property value to property of another class in wpf mvvm

Question

I want to pass SecondViewModel SecondProperty value to ViewModel myProperty and show the value on TextBlock. i want the coding to be done in SecondViewModel. Hope it is clear.

Thanks for the help in Advance.

View:

<TextBlock Text="{Binding Path=myProperty}"/>

ViewModel:

private int _myProperty;
public int myProperty
{
    get { return _myProperty; }
    set { _myProperty = value; OnPropertyChanged("myProperty"); }
}

SecondViewModel:

private int _secondProperty;
public int SecondProperty
{
   get { return _secondProperty; }
   set { _secondProperty = value; OnPropertyChanged("SecondProperty"); }
}

Answer 1

From your comment, assuming that ViewModel holds a collection of SecondViewModel items, you need to set the PropertyChangedEvent for each instance of SecondViewModel to trigger ViewModel.myProperty to refresh. e.g. ...

public class ViewModel
{
    private List<SecondViewModel> _secondViewModels = new List<SecondViewModel>();

    public IEnumerable<SecondViewModel> SecondViewModels => _secondViewModels;

    public int myProperty => _secondViewModels.Sum(vm => vm.SecondProperty);

    public void AddSecondViewModel(SecondViewModel vm)
    {
        _secondViewModels.Add(vm);
        vm.PropertyChanged += (s, e) => OnPropertyChanged(nameof(myProperty));
        OnPropertyChanged(nameof(myProperty));
    }
}

As an aside, you should never call OnPropertyChanged() with a "magic string" - use nameof() instead.

Answer 2

If you need that, you should create a VM DataSource to create a dependency in your first VM.

public class YourViewModelDataSource
 {

private ViewModel viewModel;
private SecondViewModel secondViewModel;

public YourViewModelDataSource(ViewModel viewModel, SecondViewModel secondViewModel)
    {
      this.viewModel = viewModel;
      this.secondViewModel = secondViewModel;
    }

public void CreateDataSource()
    {
      this.secondViewModel.PropertyChanged += this.OnSecondViewModelPropertyChanged;
    }

private void OnSecondViewModelPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            switch (e.PropertyName)
            {
                //As pointed in comments, you need to have the second property in your first ViewModel then do this.
                case "SecondProperty":
                    this.viewModel.myProperty = this.secondViewModel.SecondProperty;
                    break;
            }
        }
  }

When you create your VMs then use this class to create the dependency. Every time SecondViewModel SecondProperty changes, the ViewModel myProperty will be raised. I hope it's clear, let me know if you need something else