How to bind data to ViewModel wrapper and pass it to RelayCommand as parameter?

Question

I have been searching the web, but can't find the answer i need.

I have a list where the user can choose some items. In my ViewModel it looks like this:

public ObservableCollection<RentalItemVM> ChosenRentalItems {
        get { return chosenRentalItems; }
        set {
            chosenRentalItems = value;
            RaisePropertyChanged("ChosenRentalItems");
        }
    }

When the user is done selecting, he presses a button to save his collection. It works fine if I setup my RelayCommand like this:

public RelayCommand<ObservableCollection<RentalItemVM>> FinishCommand { get; set; }
private void Finish(ObservableCollection<RentalItemVM> chosenItems) {
        ...
    }

Now I need to get the UserId as well, so after some research I found out that I can't pass more than one paramater to the RelayCommand so I need a wrapper. I've made a new ViewModel:

public class OrderVM {
    public int UserId { get; set; }

    ObservableCollection<RentalItemVM> ChosenItems { get; set; }
}

and changed the RelayCommand to this:

public RelayCommand<OrderVM> FinishCommand { get; set; }
private void Finish(OrderVM order) {
        ...
    }

And i changed the XAML UserControl from this:

<StackPanel Orientation="Horizontal"
            Grid.Row="3"
            Grid.ColumnSpan="2"
            VerticalAlignment="Center"
            HorizontalAlignment="Center">
        <TextBox Text="UserId" />
        <Button Content="Finish"
                Command="{Binding FinishCommand}"
                CommandParameter="{Binding ChosenRentalItems}"
                UseLayoutRounding="False"
                FontSize="24" />
    </StackPanel>

to this:

<StackPanel Orientation="Horizontal"
            Grid.Row="3"
            Grid.ColumnSpan="2"
            VerticalAlignment="Center"
            HorizontalAlignment="Center">
        <TextBox Text="UserId"
                 x:Name="UserId" />
        <Button Content="Finish"
                Command="{Binding FinishCommand}"
                CommandParameter="{Binding OrderVM}" 
                UseLayoutRounding="False"
                FontSize="24" />
    </StackPanel>

Now this is where I get confused. I have no idea how to bind the UserId and ChosenItems. The OrderVM parameter in the RelayCommand is null, so maybe I need to instantiate it first?

Most answers that I found on the web were about displaying data from a ViewModel in a View, but almost no answers on how to post data (or I searched for the wrong things?)

Answer 1

If the command FinishCommand is in the same view model of the DataConext, you can bind UserId too.

<TextBox Text="{Binding UserId} "  x:Name="UserId"></TextBox>

and take UserId from your ViewModel instance where command is implemented.

private void Finish(ObservableCollection<RentalItemVM> chosenItems) {
    var userid = this.UserId;
    foo(userid);
}