MVVM SelectedValue from ComboBox returns Null

Question

I have a problem. I have a car class with string brand, string model,.. I also have a view with a ComboBox containing Data. When I select an item from the ComboBox "carBrand" or the ComboBox "carModel" and click on a button, I want to create a new car object. But after clicking on the button, the carBrand.SelectedValue.ToString() is delivering a Null value, same for carModel, although I selected an item from the ComboBox .

In my VMClass:

Add a1 = new Add();
c_car m1 = param as c_car; 
a1.DataContext = m1;
a1.ShowDialog();

if (a1.DialogResult.HasValue && a1.DialogResult.Value)
{ 
    m1.c_brand = a1.carBrand.SelectedValue.ToString();   //causes NullReferenceException
    m1.c_model = a1.carModel.SelectedValue.ToString();   //causes NullReferenceException 
    m1.c_year = a1.carYear.Content.ToString(); //this works perfectly
    m1.c_km = Int32.Parse(a1.carKm.Content.ToString()); //this also works properly
    //...
}

Now my View Class :

 <!--CarModel ComboBox-->
 <ComboBox x:Name="carModel" Style="{StaticResource combobox}" Grid.Column="1"
          Margin="20,15,17,14"
          ItemsSource="{Binding ModelSelectedBrand}" DisplayMemberPath="c_model" MouseLeave="carModel_MouseLeave"
          Grid.Row="2" VerticalAlignment="Center" Height="30" MouseDoubleClick="carModel_MouseDoubleClick">
 </ComboBox>

<!--CarYear EditableLabel-->
<Label x:Name="carYear" Content="{Binding ElementName=carModel, Path=SelectedValue.c_year}" Margin="20,14,17,14" 
       Style="{StaticResource EditableLabelStyle}" Foreground="White"
       Grid.Column="1" Grid.Row="5"  VerticalAlignment="Center" Height="30">
</Label>

<!--CarKM EditableLabel-->
    <Label x:Name="carKm"
       Content="{Binding ElementName=carModel, Path=SelectedItem.c_km}" Style="{StaticResource EditableLabelStyle}"
       Margin="20,14,17,14" 
       Grid.Column="1" Grid.Row="3" Foreground="White" VerticalAlignment="Center" Height="30">
</Label> 

I hope someone can help me fixing this issue.

Thanks in advance!

Answer 1

So the simple answer (I think, I haven't tested yet) is that there isn't a SelectedValuePath set on your ComboBox (As stated by vesan in the comments). This means that SelectedValue will always be null .

You could use SelectedItem to return the selected Car and then get the property from that or just set the SelectedValuePath on the ComboBox .

Now, this could of course be made better by using bindings but that is up to you whether you want to implement this.