ComboBox SelectedItem initial value not from ItemsSource - wpf

Question

I cannot understand the behavior of the combobox.
I have an edit view to edit existing Order data. Here is my VM of some part of Order data:
public class OrderDataViewModel : ViewModelBase, IOrderDataViewModel
{
private readonly ICustomersListProviderService _customersListProviderService;
private readonly Order _order;
public OrderDataViewModel(Order order, ICustomersListProviderService customersListProviderService)
{
Assign.IfNotNull(ref _order, order);
Assign.IfNotNull(ref _customersListProviderService, customersListProviderService);
}
public DateTime CreationDate
{
get { return _order.CreationDate ?? (_order.CreationDate = DateTime.Now).Value; }
set
{
if (_order.CreationDate == value) return;
_order.CreationDate = value;
OnPropertyChanged();
}
}
public Customer Customer
{
get { return _order.Customer; }
set
{
if (_order.Customer == value) return;
_order.Customer = value;
OnPropertyChanged();
}
}
private IList<Customer> _customersList;
public IList<Customer> CustomersList
{
get { return _customersList ?? (_customersList = _customersListProviderService.GetAllCustomers().ToList()); }
}
}
And XAML binding:
<ComboBox Grid.Row="2" Grid.Column="1"
SelectedItem="{Binding OrderDataViewModel.Customer}"
DisplayMemberPath="Name"
ItemsSource="{Binding OrderDataViewModel.CustomersList}"
/>
Description. Order comes from the database by the Repository, _customersListProviderService gets all customers from database also. I know that maybe it could be done better, but it's not the point of the question.
And... the issue is. After loading a window, my combobox has items collection filled (dropdown list is not empty) but the value is not set (its blank). Checking SelectedItem by code-behind results with null. I read a lot and found out, that you cannot set SelectedItem of combobox to the item that is not in ItemsSource.
Ok, my workaround was to change the Customer getter to:
public Customer Customer
{
get
{ return CustomersList.Single(c => c.Id == _order.Customer.Id); }
set
{
if (_order.Customer == value) return;
_order.Customer = value;
OnPropertyChanged();
}
}
now it works, but it does not look good to me.
Is there any better solution?

Answer 1

you can override Equals() and GetHashCode() in your entities and return Id.Equals() and Id.GetHashCode() respectively

Answer 2

I just had a similar issue within an UWP app. I was binding to a string array.
<ComboBox SelectedItem="{Binding Carrier, Mode=TwoWay}" ItemsSource="{Binding Carriers}" />
The problem solved when I changed ItemsSource to be before SelectedItem:
<ComboBox ItemsSource="{Binding Carriers}" SelectedItem="{Binding Carrier, Mode=TwoWay}" />
Just in case someone has a similar issue.

Answer 3

Perhaps adding SelectedValuePath="Name" to your xaml like below will help
<ComboBox Grid.Row="2" Grid.Column="1"
SelectedItem="{Binding OrderDataViewModel.Customer}"
DisplayMemberPath="Name"
ItemsSource="{Binding OrderDataViewModel.CustomersList}"
SelectedValuePath="Name"
/>

Answer 4

Create a ViewModel with both sets of data you need i.e set to fill ComboBox and the Record. I will use Customer and year for convenience.
class CustomerDetailsViewModel
{
public CustomertModel CutomerModel { get; set; }
public YearListModel YearList { get; set; }
public CustomerDetailsViewModel(CustomerModel _CustomerModel)
{
CustomerModel = _CustomerModel;
YearList = new YearListModel();
}
}
So I fill the combobox with a list of years and I have my selected Customer record.
<ComboBox x:Name="cbCustomerDetailsYear" Margin="128,503,0,0"
DataContext="{Binding DataContext,
ElementName=CustomerDetailsPage}"
ItemsSource="{Binding YearList.Years, Mode=OneWay}"
DisplayMemberPath="Description"
SelectedValue="{Binding CustomerModel.YearID}"
SelectedValuePath="id" />
The viewmodel is assigned to the Page Datacontext and I bind this to the Combobox.
The combobox is populated with Itemsource and Displaymember with the Year model list from my Viewmodel. The description is just a string saying 1999, 2000 or whatever
The SelectedValue is bound to the Year foreign key in the Customer record also in the ViewModel. The SelectedValuePath is the magic ingredient that binds the 2 together. So the id represents the id of the year but will be bound to the customer YearID field and set by the Combobox id value.
Hope this is clear ?!?!?!

Answer 5

I hope this would work:
Check if Customer, is already binded to another Control. If so, remove all other Bindings which involve Customer.
I had the same problem where the IsChecked property of a CheckBox was also Binded to the viewModel but using a converter to check if it's null or not. I changed its binding to a boolean and the problem was no more.