Reproduce Heisenbug

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App.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

App.xaml

<Application x:Class="ReproduceHeisenbug.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
         
    </Application.Resources>
</Application>

App.xaml.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Threading.Tasks;
using System.Windows;

namespace ReproduceHeisenbug
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
    }
}

AssemblyInfo.cs

using System.Reflection;
using System.Resources;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Windows;

// General Information about an assembly is controlled through the following 
// set of attributes. Change these attribute values to modify the information
// associated with an assembly.
[assembly: AssemblyTitle("ReproduceHeisenbug")]
[assembly: AssemblyDescription("")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("")]
[assembly: AssemblyProduct("ReproduceHeisenbug")]
[assembly: AssemblyCopyright("Copyright ©  2014")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]

// Setting ComVisible to false makes the types in this assembly not visible 
// to COM components.  If you need to access a type in this assembly from 
// COM, set the ComVisible attribute to true on that type.
[assembly: ComVisible(false)]

//In order to begin building localizable applications, set 
//<UICulture>CultureYouAreCodingWith</UICulture> in your .csproj file
//inside a <PropertyGroup>.  For example, if you are using US english
//in your source files, set the <UICulture> to en-US.  Then uncomment
//the NeutralResourceLanguage attribute below.  Update the "en-US" in
//the line below to match the UICulture setting in the project file.

//[assembly: NeutralResourcesLanguage("en-US", UltimateResourceFallbackLocation.Satellite)]


[assembly: ThemeInfo(
    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
    //(used if a resource is not found in the page, 
    // or application resource dictionaries)
    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
    //(used if a resource is not found in the page, 
    // app, or any theme specific resource dictionaries)
)]


// Version information for an assembly consists of the following four values:
//
//      Major Version
//      Minor Version 
//      Build Number
//      Revision
//
// You can specify all the values or you can default the Build and Revision Numbers 
// by using the '*' as shown below:
// [assembly: AssemblyVersion("1.0.*")]
[assembly: AssemblyVersion("1.0.0.0")]
[assembly: AssemblyFileVersion("1.0.0.0")]

Employee.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ReproduceHeisenbug
{
    public class Employee
    {
        public string Name { get; set; }
        public string Occupation { get; set; }
    }
}

EmployeeCollection.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ReproduceHeisenbug
{
    class EmployeeCollection : Collection<Employee>
    {
    }
}

MainWindow.xaml

<Window x:Class="ReproduceHeisenbug.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:obj="clr-namespace:ReproduceHeisenbug"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <ComboBox Name="cb" ItemsSource="{DynamicResource items}" HorizontalContentAlignment="Stretch">
            <ComboBox.DataContext>
                <obj:EmployeeCollection>
                    <obj:Employee Name="John" Occupation="Developer" />
                    <obj:Employee Name="Jack" Occupation="Spy" />
                </obj:EmployeeCollection>
            </ComboBox.DataContext>
            <ComboBox.Resources>
                <CompositeCollection x:Key="items">
                    <ComboBoxItem IsEnabled="False">
                        <Grid TextElement.FontWeight="Bold">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition />
                                <ColumnDefinition />
                            </Grid.ColumnDefinitions>
                            <Grid.Children>
                                <TextBlock Grid.Column="0" Text="Name"/>
                                <TextBlock Grid.Column="1" Text="Occupation"/>
                            </Grid.Children>
                        </Grid>
                    </ComboBoxItem>
                    <Separator/>
                    <CollectionContainer Collection="{Binding Source={x:Reference cb}, Path=DataContext}"/>
                </CompositeCollection>
                <DataTemplate DataType="{x:Type obj:Employee}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition />
                            <ColumnDefinition />
                        </Grid.ColumnDefinitions>
                        <Grid.Children>
                            <TextBlock Grid.Column="0" Text="{Binding Name}"/>
                            <TextBlock Grid.Column="1" Text="{Binding Occupation}"/>
                        </Grid.Children>
                    </Grid>
                </DataTemplate>
            </ComboBox.Resources>
        </ComboBox>
    </StackPanel>
</Window>

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace ReproduceHeisenbug
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

ReproduceHeisenbug.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{E19EF9A4-D77C-48C4-8879-B09C10A5A474}</ProjectGuid>
    <OutputType>WinExe</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>ReproduceHeisenbug</RootNamespace>
    <AssemblyName>ReproduceHeisenbug</AssemblyName>
    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="System" />
    <Reference Include="System.Data" />
    <Reference Include="System.Xml" />
    <Reference Include="Microsoft.CSharp" />
    <Reference Include="System.Core" />
    <Reference Include="System.Xml.Linq" />
    <Reference Include="System.Data.DataSetExtensions" />
    <Reference Include="System.Xaml">
      <RequiredTargetFramework>4.0</RequiredTargetFramework>
    </Reference>
    <Reference Include="WindowsBase" />
    <Reference Include="PresentationCore" />
    <Reference Include="PresentationFramework" />
  </ItemGroup>
  <ItemGroup>
    <ApplicationDefinition Include="App.xaml">
      <Generator>MSBuild:Compile</Generator>
      <SubType>Designer</SubType>
    </ApplicationDefinition>
    <Page Include="MainWindow.xaml">
      <Generator>MSBuild:Compile</Generator>
      <SubType>Designer</SubType>
    </Page>
    <Compile Include="App.xaml.cs">
      <DependentUpon>App.xaml</DependentUpon>
      <SubType>Code</SubType>
    </Compile>
    <Compile Include="Employee.cs" />
    <Compile Include="EmployeeCollection.cs" />
    <Compile Include="MainWindow.xaml.cs">
      <DependentUpon>MainWindow.xaml</DependentUpon>
      <SubType>Code</SubType>
    </Compile>
  </ItemGroup>
  <ItemGroup>
    <Compile Include="AssemblyInfo.cs">
      <SubType>Code</SubType>
    </Compile>
    <AppDesigner Include="Properties\" />
  </ItemGroup>
  <ItemGroup>
    <None Include="App.config" />
  </ItemGroup>
  <ItemGroup>
    <Folder Include="Properties\" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
       Other similar extension points exist, see Microsoft.Common.targets.
  <Target Name="BeforeBuild">
  </Target>
  <Target Name="AfterBuild">
  </Target>
  -->
</Project>