Problem in binding two ComboBoxes

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WPF

LINQ

combobox

Binding

Hi,
I’m trying to do something quite simple, I believe, but am very new to WPF and can’t figure it out. I want to display bank account details of a specific employee (which I select separately), and I want to let the user change those details if needed. My problem concerns specifically the following 3 tables:

Banks:
- BankID (PK)
- CountryID
- BankName
- ...

Countries:
- CountryID (PK)
- CountryName
- ...

BankAccounts:
- BankAccountID (PK)
- BankID
- ...

On my application’s MainWindow I have 2 ComboBoxes. I want to load the first one (cmbBankCountry) with a list of all the countries from ‘Countries’ table, and then automatically select the country which corresponds to the employee’s bank account.

Similarly, I want the second ComboBox (cmbBanks) to be loaded with the names of all the banks that are associated with the selected country, and here too, select automatically the bank associated with that employee’s bank account.

Of course, if the user changes the selection of the country for that bank account, the second ComboBox should be reloaded and display only the banks associated with the newly selected country.

All the data comes from an SQL database via Linq-To-SQL classes (dbml), which I generated to represent the database tables. I.e. I have a Bank class, a Country class, a BankAccount class, etc. defined in a DataContext that I declared as a Private Shared variable of Application. I also created a ReadOnly property called DBMain to access it.

So far I tried to tackle this task as follows:
To load the first Combobox with all the countries I placed the following code in the MainWindow_Loaded event:

Dim lstCountries As List(Of Country) = Application.DBMain.GetCountries()
cmbBankCountry.ItemsSource = lstCountries
cmbBankCountry.DisplayMemberPath = "CountryName"
cmbBankCountry.SelectedValuePath = "CountryID"

The GetCountries function looks like this:

Public Function GetCountries() As List(Of Country)

            Dim lstCountry As List(Of Country) = (From c In Application.DBMain.Countries
                                                                      Select c
                                                                    ).ToList
            Return lstCountry

End Function

To load the second Combobox with the corresponding banks, I created an event that is triggered when the first Combobox’s selection (country) is changed:

Private Sub DisplayBanksForCountry()

        Dim lstBanks As List(Of Bank) = Application.DBMain.GetBanks(cmbBankCountry.SelectedValue)
        cmbBanks.ItemsSource = lstBanks
        cmbBanks.DisplayMemberPath = "BankName"
        cmbBanks.SelectedValuePath = "BankID"

End Sub

And the GetBanks function looks like this:

Public Function GetBanks(inCountryID As Integer) As List(Of Bank)

            Dim lstBanks As List(Of Bank) = (From b In Application.DBMain.Banks
                                             Where b.CountryID = inCountryID
                                                           Select b).ToList

            Return lstBanks

End Function

Finally, the definition of the two ComboBoxes in the XAML file looks like this:

<ComboBox x:Name="cmbBankCountry" MinWidth="120" Grid.Row="0" Grid.Column="1" SelectedValue="{Binding Path=b.BankCountryID}" SelectionChanged="DisplayBanksForCountry"/>

<ComboBox x:Name="cmbBanks" MinWidth="120" Grid.Row="1" Grid.Column="1" SelectedValue="{Binding Path=b.BankID}"/>

In this “solution” the first ComboBox is working just fine. It is loaded with all the countries and shows automatically the country of the employee’s bank account.

However, when the Window is first loaded, the second ComboBox is empty. It is loaded with banks only after I change the selection of the first ComboBox. Obviously, this is what I asked it to do, but I was assuming (and hoping…) that the SelectionChanged event of the first ComboBox (cmbBankCountry) will occur when the ComboBox is set to display the account’s country by Binding. Unfortunately, that is not the case.

I guess that there is a very elegant way to do it via Binding between the two ComboBox, but my brain is still wired to the old Windows Forms, events oriented way of doing things.

Thank a lot in advance for any help!