<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:syncfusion="http://schemas.syncfusion.com/wpf">
    <Grid>
        <syncfusion:ComboBoxAdv Height="30" Width="200"
                                HorizontalAlignment="Center"
                                VerticalAlignment="Center">
            <syncfusion:ComboBoxItemAdv Content="Denmark" />
            <syncfusion:ComboBoxItemAdv Content="New Zealand" />
            <syncfusion:ComboBoxItemAdv Content="Canada" />
            <syncfusion:ComboBoxItemAdv Content="Russia" />
            <syncfusion:ComboBoxItemAdv Content="Japan" />
        </syncfusion:ComboBoxAdv>
    </Grid>
</Window>

// Model
public class Country
{
    public string Name { get; set; }
    public string Code { get; set; }
}

// ViewModel
public class ViewModel : INotifyPropertyChanged
{
    private ObservableCollection<Country> countries;
    public ObservableCollection<Country> Countries
    {
        get { return countries; }
        set { countries = value; OnPropertyChanged(nameof(Countries)); }
    }

    public ViewModel()
    {
        Countries = new ObservableCollection<Country>
        {
            new Country { Name = "Denmark", Code = "DK" },
            new Country { Name = "New Zealand", Code = "NZ" },
            new Country { Name = "Canada", Code = "CA" }
        };
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string name) =>
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}

<Window.DataContext>
    <local:ViewModel />
</Window.DataContext>
<syncfusion:ComboBoxAdv ItemsSource="{Binding Countries}"
                        DisplayMemberPath="Name"
                        Height="30" Width="200"/>

<syncfusion:ComboBoxAdv AllowMultiSelect="True"
                        IsEditable="True"
                        EnableToken="True"
                        ItemsSource="{Binding Items}"
                        DisplayMemberPath="Name"
                        Height="Auto" Width="300"/>

<syncfusion:ComboBoxAdv IsEditable="True"
                        AutoCompleteMode="Suggest"
                        ItemsSource="{Binding Items}"
                        DisplayMemberPath="Name"
                        Height="30" Width="200"/>

<syncfusion:ComboBoxAdv AllowMultiSelect="True"
                        DefaultText="Select items..."
                        SelectedValueDelimiter=" | "
                        ItemsSource="{Binding Items}"
                        DisplayMemberPath="Name"
                        Height="30" Width="300"/>

<syncfusion:ComboBoxAdv AllowMultiSelect="True"
                        AllowSelectAll="True"
                        EnableOKCancel="True"
                        ItemsSource="{Binding Items}"
                        DisplayMemberPath="Name"
                        Height="30" Width="250"/>