Advice on viewmodel and inter-connected combo boxes - wpf

Question

I am having a few struggles on how to wire up my interconnected comboboxes when using MVVM. I have a DTO that represents an Order which contains a CustomerId and an OrderTypeId. Thesere are then wrapped inside an OrderViewModel. I also have an EditOrderViewModel which loads from a db a list of Customers.
What I would like to do is Load an Order from the DB (similar to the Load function) choose the right item in the ComboBox (the items source of which is a List, display the name of the selected customer in a text block to the right of the combobox and finally load the list of OrderTypes that belong to that Customer in the next combobox and again select the correct OrderType and display the OrderTypeName in a TextBlock to the right.
I have managed to get some of the behaviour to work when I use SelectedItem from the combobox but this is only when I select the item manually as I am not sure how in my viewmodel I can convert Order.CustomerId (type int) into the correct SelectedItem (type CustomerDTO). Below is some code which shows generally what I am trying to achieve whilst using in-memory datasources. Thanks Alex
<ComboBox Height="25" Width="150" ItemsSource="{Binding Path=Customers}" SelectedValue="{Binding Path=Order.CustomerId}" SelectedValuePath="Id">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=Code}"></TextBlock>
<TextBlock Text="{Binding Path=Name}"></TextBlock>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
<TextBlock Text="{Binding Path=Name,Mode=OneWay,NotifyOnSourceUpdated=True}"></TextBlock>
public class EditOrderViewModel : VMBase
{
public OrderViewModel Order{get;set;}
public void Load()
{
Order = new OrderViewModel(new OrderDto{CustomerId=1,OrderTypeId=2});
Order.PropertyChanged += MainWindowViewModel_PropertyChanged;
}
public EditOrderViewModel()
{
Order = new OrderViewModel(new OrderDto());
Order.PropertyChanged += OrderViewModel_PropertyChanged;
Customers = new List<CustomerDto> {
new CustomerDto{ Id = 1, Code = "ACME", Name = "ACME CORP" },
new CustomerDto{ Id = 2, Code = "MSFT", Name="MICROSOFT CORP" },
new CustomerDto{ Id = 3, Code = "APP", Name = "APPLE" }};
OrderTypes = new List<OrderTypeDto>{
new OrderTypeDto{OrderTypeId=1, CustomerId =1, Name = "Cake Order"},
new OrderTypeDto{OrderTypeId=2, CustomerId =1, Name = "Sandwich Order"},
new OrderTypeDto{OrderTypeId=3, CustomerId =2, Name = "Chocolate Order"},
new OrderTypeDto{OrderTypeId=4, CustomerId =2, Name = "Bread Order"},
new OrderTypeDto{OrderTypeId=5, CustomerId =3, Name = "Drinks Order"}};
}
void OrderViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
switch (e.PropertyName)
{
case "OrderTypeId":
break;
case "SelectedCustomer":
break;
default:
break;
}
}
public List<OrderTypeDto> OrderTypes { get; set; }
public List<CustomerDto> Customers { get; set; }
}
public class OrderDto
{
public int CustomerId { get; set; }
public int OrderTypeId { get; set; }
}
public class CustomerDto
{
public int Id { get; set; }
public string Name { get; set; }
public string Code { get; set; }
}
public class OrderViewModel : VMBase
{
private OrderDto _orderDto;
private string _customerName;
public OrderViewModel(OrderDto orderDto)
{
_orderDto = orderDto;
}
public int CustomerId {
get { return _orderDto.CustomerId; }
set
{
_orderDto.CustomerId = value;
RaisePropertyChanged("CustomerId");
}
}
public string CustomerName {
get { return _customerName; }
set {_customerName = value;
RaisePropertyChanged("CustomerName");
}
}
public int OrderTypeId
{
get { return _orderDto.OrderTypeId; }
set
{
_orderDto.OrderTypeId = value;
RaisePropertyChanged("OrderTypeId");
}
}
}

Answer 1

Don't set ComboBox.SelectedValue in your XAML binding. You should be binding ComboBox.SelectedItem to your model so you can have the CustomDTO easily available. You should add a property to your OrderViewModel called Customer (of type CustomerDTO) instead of trying to recreate the CustomerDTO using several properties (CustomerID, CustomerName, etc.).