WPF CommandParameter binding and canExecute

Question

I have a template for treeView item:

<HierarchicalDataTemplate x:Key="RatesTemplate">
    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding Path=ID}"/>
                        <Button CommandParameter="{Binding Path=ID}" 
                                Command="{Binding ElementName=CalcEditView, Path=DataContext.Add}">Add</Button>                            
    </StackPanel>
</HierarchicalDataTemplate>

As a DataContext I have linq entity with ID not null field.

The problem is: if I use DelegateCommand 'Add' with CanExecutedMethod:

AddRate = new DelegateCommand<int?>(AddExecute,AddCanExecute);

its called only once and parameter is null (while textBlock shows proper ID value). CanExecute is called before ID property is called (checked with debugger). Seems like before binding to actual parameter wpf is invoking canExecute and forgets about it. Once binding finished and proper value loaded it doesn't call CanExecute again.

As a workaround I can use command with only execute delegate:

Add = new DelegateCommand<int?>(AddExecute);

AddExecute is invoked with correct ID value and is working perfectly. But I still want to use CanExecute functionality. Any ideas?

Answer 1

In this scenario, it's better to call the RaiseCanExecuteChanged() on the property used as a parameter for the Command. In your case, it would be the ID property in your ViewModel (Or whichever DataContext you're using).

It would be something like this:

private int? _id;
public int? ID
{
    get { return _id; }
    set
    {
        _id = value;
        DelegateCommand<int?> command = ((SomeClass)CalcEditView.DataContext).Add;
        command.RaiseCanExecuteChanged();
    }
}

The effect will be the same as your solution, but it keeps the Command logic out of code-behind.