如何在 WPF MVVM 中的 combobox 中输入默认选择值

Question

I have a combobox in WPF MVVM and I have 3 value in combobox. Values are: AB-All I want to "All" is default selection in combobox how can I do it?

combobox in my xaml:

<ComboBox  ItemsSource="{Binding Locations}" SelectedItem ="{Binding SelectedLocation}">
                    <i:Interaction.Triggers>
                        <i:EventTrigger EventName="SelectionChanged">
                            <i:InvokeCommandAction Command="{Binding LocationFilterCommand}"/>
                        </i:EventTrigger>
                    </i:Interaction.Triggers>
                </ComboBox>

Locations observable collection in my viewmodel:

private ObservableCollection<string> _locations;
        public ObservableCollection<string> Locations
        {
            get { return _locations; }
            set
            {
                _locations = value;
                OnPropertyChanged("Locations");
            }
        }

        Locations = new ObservableCollection<string>()
        {
            "All","A","B"
        };

thank you for all your helps.

Answer 1

View

<ComboBox
    ItemsSource="{Binding Locations}"
    SelectedItem="{Binding SelectedLocation}" />

ViewModel [using MVVMLight]

public class MainViewModel: ViewModelBase
{
    public MainViewModel()
    {
        SelectedLocation = Locations.First();
    }

    public IEnumerable<string> Locations => new[] {"All", "A", "B"};

    private string _selectedLocation;

    public string SelectedLocation
    {
        get => _selectedLocation;
        set
        {
            if (Set(nameof(SelectedLocation), ref _selectedLocation, value))
            {
                // Update location filter here
            }
        }
    }
}

There's no need to use an ObservableCollection for Locations unless you plan to add more elements to the collection at some point.