Question

When I have background colors on the items in an ItemsControl, and set the margins to 0, WPF leaves hairlines between the items as if the ItemsControl wrapper plumbing is taking up a minute amount of space. I checked the visual tree with Snoop and all margins are set to 0,0,0,0.

What are causing these lines and how can I avoid them?

XAML:

<DockPanel>

    <StackPanel DockPanel.Dock="Top" Orientation="Horizontal" Background="Yellow" >
        <ItemsControl ItemsSource="{Binding CustomerList}">

            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <StackPanel Background="DarkGreen">
                        <TextBlock Text="{Binding LastName}"/>
                    </StackPanel>
                </DataTemplate>
            </ItemsControl.ItemTemplate>

            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <DockPanel Margin="10"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>

        </ItemsControl>
    </StackPanel>

</DockPanel>

Code-Behind:

using System.Windows;
using System.ComponentModel;
using System.Collections.ObjectModel;
namespace TestItemsControl2938
{
    public partial class Window1 : Window, INotifyPropertyChanged
    {
        private ObservableCollection<Customer> _customerList = new ObservableCollection<Customer>();
        public ObservableCollection<Customer> CustomerList
        {
            get
            {
                return _customerList;
            }

            set
            {
                _customerList = value;
                OnPropertyChanged("CustomerList");
            }
        }

        public Window1()
        {
            InitializeComponent();
            DataContext = this;

            CustomerList.Add(new Customer { FirstName = "Jim", LastName = "Jones" });
            CustomerList.Add(new Customer { FirstName = "Joe", LastName = "Adams" });
            CustomerList.Add(new Customer { FirstName = "Jake", LastName = "Johnson" });
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;

            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

    public class Customer
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Street { get; set; }
        public string Location { get; set; }
        public string ZipCode { get; set; }
    }

}

Answer:

Here’s the fix, thanks Kent:

<ItemsControl.ItemTemplate>
    <DataTemplate>
        <StackPanel Background="#ccc" SnapsToDevicePixels="True">
            <TextBlock Text="{Binding LastName}"/>
        </StackPanel>
    </DataTemplate>
</ItemsControl.ItemTemplate>

Answer 1

It may be WPF inferring a certain amount of transparency because the items don’t fall on exact pixel boundaries. Try playing with the SnapsToDevicePixels property on the item containers and see if that helps.