Combobox doesn't display SelectedIndex On Startup in WPF

Question

I am working on a WPF app where I need to work with comboboxes. I need to add items in the combobox and set the selectedindex in xaml. I have done it successfully but SelectedIndex doesn't appear when I run the application. Here is my code:

XAML:

<ComboBox Grid.Column="0" ItemsSource="{Binding FrequencyList}" SelectedItem="{Binding SelectedFrequencyList, Mode=TwoWay}" SelectedIndex="0" Name="comboBox1" />
<ComboBox Grid.Column="2" ItemsSource="{Binding ModesList}" SelectedItem="{Binding SelectedModesList, Mode=TwoWay}" SelectedIndex="2" Name="comboBox2" />

ViewModel:

public ObservableCollection<string> _FreqList;
_FreqList = new ObservableCollection<string>();
public ObservableCollection<string> _CodecModes;
_CodecModes= new ObservableCollection<string>();

public ViewModel()
{
        _FreqList.Add("8 kHz");
        _FreqList.Add("11.025 kHz");
        _FreqList.Add("12 kHz");
        _FreqList.Add("14.7 kHz");
        _FreqList.Add("16 kHz");
        _FreqList.Add("22.050 kHz");
        _FreqList.Add("24 kHz");
        _FreqList.Add("29.4 kHz");
        _FreqList.Add("32 kHz");
        _FreqList.Add("44.100 kHz");
        _FreqList.Add("48 kHz");
        _FreqList.Add("88.2 kHz");
        _FreqList.Add("96 kHz");
        _FreqList.Add("undef");

        _CodecModes.Add("None");
        _CodecModes.Add("A Loop");
        _CodecModes.Add("DSP");
        _CodecModes.Add("I2S");            
}

public ObservableCollection<string> FrequencyList
    {
        get { return _FreqList; }
        set
        {
            _FreqList = value;
            OnPropertyChanged("FrequencyList");
        }
    }

    /// <summary>
    /// Selected Frequency List
    /// </summary>
    private string _selectedFrequencyList;
    public string SelectedFrequencyList
    {
        get { return _selectedFrequencyList; }
        set
        {
            _selectedFrequencyList = value;
            int Listvalue = FrequencyList.IndexOf(_selectedFrequencyList);
            int ListFinalVal = Listvalue + 1;
            SelectedFreq(ListFinalVal);
            OnPropertyChanged("SelectedFrequencyList");
        }
    }

    public void SelectedFreq(int Select)
    {            
        int cmd = 0;
        int numBytes = 0;           

        cmd = ((0x8F00 & 0x7F00) | (m_slot & 0xFF));
        sendBuf[numBytes++] = Convert.ToByte(Select - 1);
    }

    public ObservableCollection<string> ModesList
    {
        get { return _CodecModes; }
        set
        {
            _CodecModes = value;
            OnPropertyChanged("ModesList");
        }
    }

    /// <summary>
    /// Selected Modes List
    /// </summary>
    private string _selectedModesList;
    public string SelectedModesList
    {
        get { return _selectedModesList; }
        set
        {
            _selectedModesList = value;
            int Modevalue = ModesList.IndexOf(_selectedModesList);
            int ModeFinalvalue = Modevalue + 1;
            SelectedMode(ModeFinalvalue);
            OnPropertyChanged("SelectedModesList");
        }
    }

    public void SelectedMode(int Select)
    {
        int cmd = 0;
        int numBytes = 0;           

        cmd = ((0x8F00 & 0x7F00) | (m_slot & 0xFF));
        sendBuf[numBytes++] = Convert.ToByte(Select - 1);
    }

Even though I have set SelectedIndex=1 in my xaml. When I run the app, the combobox has set of items but doesn't display the selected-index. Am I missing something here???

Answer 1

Setting SelectedIndex=1 conflicts withyour binding if SelectedItem with initial null value.

You can chnage the binding mode for SelectedItem to OneWayToSource so that the value in the view model won't be used for setting the selected item:

<ComboBox Grid.Column="0" ItemsSource="{Binding FrequencyList}" SelectedItem="{Binding SelectedFrequencyList, Mode=OneWayToSource}" SelectedIndex="1" Name="comboBox1" />

Alternatively you can initialize SelectedFrequencyList in view model to the desired frequency:

public ViewModel()
{
    _FreqList.Add("8 kHz");
    _FreqList.Add("11.025 kHz");
    _FreqList.Add("12 kHz");
    _FreqList.Add("14.7 kHz");
    _FreqList.Add("16 kHz");
    _FreqList.Add("22.050 kHz");
    _FreqList.Add("24 kHz");
    _FreqList.Add("29.4 kHz");
    _FreqList.Add("32 kHz");
    _FreqList.Add("44.100 kHz");
    _FreqList.Add("48 kHz");
    _FreqList.Add("88.2 kHz");
    _FreqList.Add("96 kHz");
    _FreqList.Add("undef");

    SelectedFrequencyList = _FreqList[1];
}

In this case you don't need to set the selected index for the combo box at all:

<ComboBox Grid.Column="0" ItemsSource="{Binding FrequencyList}" SelectedItem="{Binding SelectedFrequencyList, Mode=TwoWay}" Name="comboBox1" />