<ComboBox Name="cmbMainList" Margin="5,5,5,5" Width="250px" SelectionChanged="cmbMainList_SelectionChanged" SelectedItem="{Binding SelectedItem}" />

cmbMainList.ItemsSource = this.mainList;

private List<String> mainList= new List<string>();

cmbMainList.SelectedValue = this.mainList.Where(n => n.Equals(".....")).First();

 public partial class MainWindow : Window
    {
        private List<string> _names;
        public String SimpleNames { get; set; }

        public List<string> Names
        {
            get { return _names; }
            set { _names = value; }
        }

        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }

        private void btnSelect_Click(object sender, RoutedEventArgs e)
        {
            cmbBox.SelectedItem  = Names[1];
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            _names = new List<string>() { "Номер №1", "Номер №2", "Номер №3" }; 
        }
    }

<Window x:Class="WpfTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" Loaded="Window_Loaded">
    <StackPanel Loaded="StackPanel_Loaded">
        <ComboBox 
            ItemsSource="{Binding Names}"
            SelectedItem="{Binding SimpleNames, Mode=TwoWay}" 
          IsSynchronizedWithCurrentItem="True" 
            Name="cmbBox"
            Margin="5,5,5,5" 
            Width="250px" />
    </StackPanel>
</Window>

      public List<string> Names
        {
            get { return new List<string>() { "Номер №1", "Номер №2", "Номер №3" }; ; }
            set { _names = value; }
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            _names = new List<string>() { "Номер №1", "Номер №2", "Номер №3" };
            DataContext = this;
        }