WPF

<Window x:Class="ComboBoxDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        DataContext="{Binding RelativeSource={RelativeSource self}}"
        Title="MainWindow"
        Height="350" 
        Width="525">
    
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="40" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <ComboBox ItemsSource="{Binding MyData}" 
                  DisplayMemberPath="Value1" 
                  SelectedItem="{Binding YourSelectedItem}" Grid.ColumnSpan="3" />

        <TextBox Text="{Binding YourSelectedItem.Value1}" Grid.Row="1" />
        <TextBox Text="{Binding YourSelectedItem.Value2}" Grid.Row="1" Grid.Column="1" />
        <TextBox Text="{Binding YourSelectedItem.Value3}" Grid.Row="1" Grid.Column="2" />
    </Grid>
</Window>

using System.Collections.ObjectModel;
using System.ComponentModel;

namespace ComboBoxDemo
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : INotifyPropertyChanged
    {
        public ObservableCollection<MyDataObject> MyData { get; set; }

        private MyDataObject yourSelectedItem;
        public MyDataObject YourSelectedItem
        {
            get { return yourSelectedItem; }
            set { yourSelectedItem = value; OnPropertyChanged("YourSelectedItem"); }
        }

        public MainWindow()
        {
            MyData = new ObservableCollection<MyDataObject>();

            InitializeComponent();

            AddData(new MyDataObject {Value1 = 1, Value2 = "string 1", Value3 = 1});
            AddData(new MyDataObject {Value1 = 2, Value2 = "string 2", Value3 = 2});
            AddData(new MyDataObject {Value1 = 3, Value2 = "string 3", Value3 = 3});
        }

        public void AddData(MyDataObject obj)
        {
            MyData.Add(obj);
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void OnPropertyChanged(string info)
        {
            if(PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

namespace ComboBoxDemo
{
    public class MyDataObject
    {
        public int Value1 { get; set; }
        public string Value2 { get; set; }
        public int Value3 { get; set; }
    }
}

private Point GetElementLocation<T>(Window window, T control) where T : UIElement
{
  GeneralTransform xform = control.TransformToAncestor(window);
  return xform.Transform(new Point(0,0));
}

private Point GetElementLocation<T>(Window window, T control) where T : UIElement
{
  Vector vector = VisualTreeHelper.GetOffsetAt(control);
  return new Point(vector.X, vector.Y);
}

Rect selectionRect = new Rect(_mouseDownPos, mouseUpPos);

foreach (var editor in Editors)
{
    var bounds = VisualTreeHelper.GetDescendantBounds(editor);

    if (selectionRect.IntersectsWith(bounds))
    {
        editor.IsSelected = true;
    }
    else
    {
        editor.IsSelected = false;
    }
}