Unable to set SelectedValue in Combobox using DataTrigger

Question

I have a comboBox that has a datatrigger that set its SelectedIndex based on a .NET Property's value that in the VM. My problem is that I can't get the setter to set the Selected Index.

The ItemSource is based on a enum array. The DataContext of the Window is the VM which has the Modulation, and Bandwidth properties.

I'm new to WPF so I'm sure I'm not understanding binding correctly, but I'm pulling my hair out! Thanks for your help in advance.

Here's the Style.

    <Style x:Key="BWCombBoxStyle" TargetType="{x:Type ComboBox}" BasedOn="{StaticResource {x:Type ComboBox}}">
        <Style.Triggers>
            <Trigger Property="Validation.HasError" Value="true">
                <Setter Property="ToolTip"
                        Value="{Binding RelativeSource={x:Static RelativeSource.Self},
                        Path=(Validation.Errors).CurrentItem.ErrorContent}"/>
            </Trigger>
            <DataTrigger 
                Binding="{Binding Modulation}" Value="P25">
                <Setter Property="SelectedIndex" Value="2"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>

Here's the ComboBox:

   <ComboBox Name="bandwidth" 
             Height="Auto" Width="70"
             Style="{StaticResource BWCombBoxStyle}"
             ItemsSource="{Binding BandwidthOptions, Mode=OneWay, ValidatesOnDataErrors=true, NotifyOnValidationError=true, UpdateSourceTrigger=PropertyChanged}"
             SelectedValue="{Binding IFBandwidth, Mode=TwoWay, ValidatesOnDataErrors=True, 
             NotifyOnValidationError=True, UpdateSourceTrigger=PropertyChanged}"/>

Here are the .Net Properties in my VM:

    public TMod Modulation
    {
        get { return modulation_; }
        set { modulation_ = value; NotifyPropertyChanged("Modulation"); }
    }

    public Channel.TBnd IFBandwidth
    {
        get { return chan_.IFBandwidth; }
        set
        {
            chan_.IFBandwidth = value; 
            NotifyPropertyChanged("IFBandwidth"); 
        }
    }

    public Channel.TBnd[] BandwidthOptions
    {
        get
        {
            return (Channel.TBnd[])System.Enum.GetValues(typeof(Channel.TBnd));
        }
    }

Here are the enums:

    public enum TMod
    {
        FM = 0,
        AM = 1,
        P25 = 2,
        TRK = 3
    }

    public enum TBnd
    {
        Std = 0,
        Nar = 1,
        Wide = 2,
        XWide = 3
    }

Answer 1

Change your ComboBox binding to use SelectedValue instead of SelectedPath. That will properly set the IFBandwidth view model property when the value is changed.

What exactly is the trigger going to be used for? It may be a better option to change your Modulation property to be something like this...

public TMod Modulation
{
    get { return modulation_; }
    set
    {
        modulation_ = value; 
        NotifyPropertyChanged("Modulation");

        if( modulation == TMod.P25 )
        {
            IFBandwith = TBand.Wide;
        }
    }
 }