Multi tool use

How I can implement a function depending Checkbox changes into Datagrid?

I have a requierement in WPF, I have a ViewModel with a long property called TotalImport and a ObservableCollection called CuentasxCobrar
CxC is a class with a long property called Import, and a bool property called Selected
The CuentasxCobrar is the ItemSource of a Datagrid into my View
I must calculate TotalImport as the sum of Import for each element with Selected on true
My ViewModel Works fine, but the function that calculate TotalImport never is called

#region CuentasxCobrar
private ObservableCollection<CxC> _cuentasxcobrar;
public ObservableCollection<CxC> CuentasxCobrar
{
set
{
_cuentasxcobrar = value;
**ActualizaImporteAcumulado();**
OnPropertyChanged("CuentasxCobrar");
OnPropertyChanged("ImporteAcumulado");
}
get { return _cuentasxcobrar; }
}
#endregion

#region ActualizaImporteAcumulado
private void ActualizaImporteAcumulado()
{
_importeacumulado = 0;
foreach (var item in _cuentasxcobrar)
{
_importeacumulado += item.seleccionada ? item.importepago : 0;
}
OnPropertyChanged("ImporteAcumulado");
}
#endregion


But when I made click on the Checkbox into my Datagrid, that was bound to the property Selected, the change to the Cuentas x Cobrar is not called

Investigating I find this:

How to rewrite this DataGrid MouseLeftButtonUp binding to MVVM?

But never is called the MouseLeftButtonUpCommand function

I made in my ViewModer this:

#region MouseLeftButtonCommand
private RelayCommand _mouseLeftButtonUpCommand;
public RelayCommand MouseLeftButtonUpCommand
{
get
{
return _mouseLeftButtonUpCommand
?? (_mouseLeftButtonUpCommand = new RelayCommand(
() =>
{
ActualizaImporteAcumulado(); // the handler goes here
}));
}
}

public class RelayCommand : ICommand
{
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
private Action methodToExecute;
private Func<bool> canExecuteEvaluator;
public RelayCommand(Action methodToExecute, Func<bool> canExecuteEvaluator)
{
this.methodToExecute = methodToExecute;
this.canExecuteEvaluator = canExecuteEvaluator;
}
public RelayCommand(Action methodToExecute)
: this(methodToExecute, null)
{
}
public bool CanExecute(object parameter)
{
if (this.canExecuteEvaluator == null)
{
return true;
}
else
{
bool result = this.canExecuteEvaluator.Invoke();
return result;
}
}
public void Execute(object parameter)
{
this.methodToExecute.Invoke();
}
}
#endregion


And in my View:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
........
</DataGrid>
.....
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDown" >
<i:InvokeCommandAction Command="{Binding MouseLeftButtonUpCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
</DataGrid>


What is wrong?

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