Question

I got a sample mvvm app. The UI has a textbox, a button and a combobox. when I enter something in the textbox and hit the button, the text I enter gets added to an observablecollection. The Combobox is bound to that collection. How do I get the combobox to display the newly added string automaticly?

Answer 1

As I understand correctly, you want to add an item and select it.
Here is the example how it can be done using ViewModel and bindings.

Xaml:

<StackPanel>
    <TextBox Text="{Binding ItemToAdd}"/>
    <ComboBox ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem}" />
    <Button Content="Add" Click="Button_Click"/>
</StackPanel>

ViewModel:

public class MainViewModel:INotifyPropertyChanged
{
    public ObservableCollection<string> Items { get; set; }

    public string ItemToAdd { get; set; }

    private string selectedItem;

    public string SelectedItem
    {
        get { return selectedItem; }
        set
        {
            selectedItem = value;
            OnPropertyChanged("SelectedItem");
        }
    }

    public void AddNewItem()
    {
        this.Items.Add(this.ItemToAdd);
        this.SelectedItem = this.ItemToAdd;
    }


    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

The MainViewModel has 3 properties (one for the TextBox and two other for the ComboBox) and the method AddNewItem without parameters.

The method can be triggered from a command, but there is no standard class for commands, so I will call it from the code-behind:

   ((MainViewModel)this.DataContext).AddNewItem();

So you must explicitly set an added item as selected after you add it to a collection.

Because the method OnItemsChanged of the ComboBox class is protected and can’t be used.