Solving cross-assembly WPF Resource problems once and for all

I wrote several posts about WPF weakness when it comes to sharing resources. Our use case, at my at work, where we load multiple versions of an assembly into a single app domain this problem has been felt most acutely. As I wrote previously, using the most documented way of loading resource dictionaries, unless you specify a version of an assembly when loading a resource dictionary you are running the risk of loading a wrong resource dictionary because you may have multiple versions of same library loaded into an app domain. And such problems are notoriously difficult to catch and understand.

At least now, it’s easy to solve. I am glad to announce that I found a definitive way to share resources across assembly boundaries without running into versioning issues. The key to doing this is to never load resource dictionary from another assembly using cross-assembly Uris. I did not know that there was another way until recently. All the examples of loading resource dictionaries, that I can recall, use the Source=”{PackUri}” method of loading, but there’s a way to reference resource dictionary using code.

A ResourceDictionary, just like any other class that can be defined in Xaml, can also contain the “code behind” portion, a source code file associated with the main Xaml file. If you already have resources defined in a resource dictionary somewhere in your assembly, you don’t have to change it. Instead you’ll create a brand new resource dictionary which you would use to export the resource for sharing. For example, let’s say you have a resource dictionary defining the colors of the stoplight. It lives in an assembly called ResourceSharing, in a namespace ResourceSharing.Resources in a file called Resources.xaml.


<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    
    <SolidColorBrush x:Key="Go" Color="Green"/>
    <SolidColorBrush x:Key="Stop" Color="Red"/>
    <SolidColorBrush x:Key="PrepareToStop" Color="Yellow"/>
    
</ResourceDictionary>

To export this ResourceDictionary, create a new Resource Dictionary for export. Let’s call it ResourceSharing.Resources.ExportResourceDictionary. I’ll consist of the xaml and the C# portions

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
                    x:Class="ResourceSharing.Resources.ExportResourceDictionary"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Resources.xaml"/>
    </ResourceDictionary.MergedDictionaries>
    
</ResourceDictionary>

Notice the Class definition on line 2.

The code behind class looks like this

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

namespace ResourceSharing.Resources
{
    public partial class ExportResourceDictionary
    {
        //Expose it as singleton to avoid multiple instances of this dictionary
        private static readonly ExportResourceDictionary _instance = new ExportResourceDictionary();

        public static ExportResourceDictionary Instance
        {
            get { return _instance; }
        }

        public ExportResourceDictionary()
        {
            InitializeComponent();
        }
    }
}

Let’s also create a namespace we can use to make it easier to reference these resources from another assembly using namespaces

using System.Windows.Markup;

[assembly:XmlnsDefinitionAttribute("http://my.schemas.com/web/resources", "ResourceSharing.Resources")]

How to Share

Here’s how to consume the resources exported from the ResourceSharingAssembly. For this example, I had an executable assembly called ResourceSharingClient, where I am referencing exported resource right in the Main Window. Notice the use of the exported namespaces in line 2 and the way that I am referencing a static instance of the ExportResourceDictionary. Such references are version-proof because .NET knows the version of the dependent assemblies. There’s no need to dynamically discover the dependent assembly and the code actually looks a lot neater too. I know that such approach requires a bit more work, but the final result is a lot safer. I only wished that such approach had been advertised by Microsoft WPF evangelists from the beginning.

<Window
        x:Class="ResourceSharingClient.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:rs="http://my.schemas.com/web/resources"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
        mc:Ignorable="d"
        Title="MainWindow" d:DesignWidth="111.5" d:DesignHeight="109.5" Width="120" Height="130">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <x:Static Member="rs:ExportResourceDictionary.Instance"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Border BorderBrush="Black" BorderThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center" Padding="2" CornerRadius="2" Background="Silver">
        <StackPanel Orientation="Vertical">
            <Ellipse Width="20" Height="20" Fill="{DynamicResource Stop}" Stroke="Black" StrokeThickness="1"/>
            <Ellipse Width="20" Height="20" Fill="{DynamicResource PrepareToStop}" Stroke="Black" StrokeThickness="1"/>
            <Ellipse Width="20" Height="20" Fill="{DynamicResource Go}"  Stroke="Black" StrokeThickness="1"/>
        </StackPanel>
    </Border>
</Window>
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Solving the WPF Resource Key Collision

In my previous post about WPF Resource Keys, I talked about how WPF handles resource resolution when multiple resources with same key end up in loaded into same ResourceDictionary via merging.  In this post, I will present a solution.

So, apparently, Microsoft thought about it and provided a solution in a form of a class class called System.Windows.ComponentResourceKey.  This class is derives from a base class called System.Windows.ResourceKey, which is used inside the WPF resource resolution code.

So what does it do and how does work?

Normally, the WPF resource resolution logic assumes that all keys are objects used to identify resources in a ResourceDictionary.  Unlike a regular dictionary, a ResourceDictionary, technically, supports having multiple resources added with same key, but during the lookup operation, only the last resource added for a specific key will be returned.  So during the ‘merge’ operation, the new resources will overlay the existing ones if the keys clash, and during the ‘unmerge’ operation (ResourceDIctionary.MergedDictionaries.Remove()) the previously covered resource will emerge as ‘active’, so all the resources referenced via DynamicResouce will update.

The above gets more interesting if the key is of type System.Windows.ResourceKey.  If so, the resource resolution logic will only look at the resources which live in a dictionaries defined in the assembly referenced by the ResouceKey.  Moreover, when such set of dictionaries is found, the instance of the key is compared against other instances of the key to see if there’s a match, and because the System.Windows.ComponentResourceKey class overrides Equals and GetHashCode, the rules of equality take into consideration a bunch of parts, not just string value.

Here’s the code of the Equals and GetHashCode methods:

public override bool Equals(object o)
{
    var componentResourceKey = o as ComponentResourceKey;
    if (componentResourceKey == null)
    {
        return false;
    }
    // Ether neither objects define the TypeInTargetAssembly, 
    // or if both are defined, then make sure they are same
    if (!((componentResourceKey._typeInTargetAssembly != null) ? 
              componentResourceKey._typeInTargetAssembly.Equals(this._typeInTargetAssembly) :
             (this._typeInTargetAssembly == null)))
    {
        return false;
    }
    //If resourceId is null make sure they are both null
    if (componentResourceKey._resourceId == null)
    {
        return this._resourceId == null;
    }
    //otherwise do a simple equals
    return componentResourceKey._resourceId.Equals(this._resourceId);
}

public override int GetHashCode()
{  
   //use both, the type and the resouceid to generate a complex hashcode
   return ((this._typeInTargetAssembly != null) ? this._typeInTargetAssembly.GetHashCode() : 0) 
               ^ ((this._resourceId != null) ? this._resourceId.GetHashCode() : 0);
}

So when creating a reusable WPF component make sure you use the ComponentResourceKey class for your resource keys, do not just use strings and, obviously, refer to such resources using the ComponentResourceKey as well.  You can designate a single marker type (an empty public class) that lives in your assembly as a TypeInTargetAssembly to make it easier to reference your assembly when both, defining and referencing a resource.  This StackOverflow question showcases the use: http://stackoverflow.com/questions/337803/how-do-i-get-the-actual-resource-from-a-componentresourcekey.


//Exposing
public class Resources
{
    public static ComponentResourceKey BaseControlStyleKey
    {
        get
        {
            return new ComponentResourceKey(typeof(Resources), "BaseControlStyle");
        }
    }
}


//Using
myTextBox.Style = 
        Application.Current.TryFindResource(Resources.BaseControlStyleKey)
        as Style;



<!--Exposing -->
<Style x:Key="{ComponentResourceKey TypeInTargetAssembly={x:Type local:Resources}, ResourceId=BaseControlStyle}" TargetType="{x:Type FrameworkElement}">
    <Setter Property="Margin" Value="4,4,0,0" />
</Style>

<!-- Using -->
<TextBlock Style="{DynamicResource {ComponentResourceKey TypeInTargetAssembly={x:Type local:Resources}, ResourceId=BaseControlStyle}}"/>

<!-- Using in xaml by relying on the static property defined above -->
<TextBlock Style="{DynamicResource {x:Static local:Resources.BaseControlStyleKey}}"/>

WPF Resource Key Collision

Our framework allows developers to create add-ins (modules), which are loaded at run time.  It’s possible for each developer to create custom resources and merge them into a Resource Dictionary on the Application object.  So what happens when two different add-ins define a resource with the same key?  How does WPF handle such key collision?

To answer this question, II created a small application which tests the ability of a single resource dictionary to load multiple merged dictionaries which have objects with same key. The code is below but I did discover that

  1. the app does not crash – WPF handles redifinition of the key gracefully
  2. the last one wins – whichever dictionary defining the entry with the same key loads last is the one that’s going to be resolved when app asks for such resource using the DynamicResource
  3. the original definition becomes effective when the override is removed – removing the duplicate definition makes the original value effective

It’s important to note that if the key already exists in the primary dictionary, the one into which other dictionaries are merged, the discovery rules will ignore any re-definition of the key, according to docs here.

Resources in a merged dictionary occupy a location in the resource lookup scope that is just after the scope of the main resource dictionary they are merged into. Although a resource key must be unique within any individual dictionary, a key can exist multiple times in a set of merged dictionaries. In this case, the resource that is returned will come from the last dictionary found sequentially in the MergedDictionaries collection. If the MergedDictionaries collection was defined in XAML, then the order of the merged dictionaries in the collection is the order of the elements as provided in the markup. If a key is defined in the primary dictionary and also in a dictionary that was merged, then the resource that is returned will come from the primary dictionary. These scoping rules apply equally for both static resource references and dynamic resource references.

<Application x:Class="Test.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="rd1.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:sys="clr-namespace:System;assembly=mscorlib"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    
    <sys:String x:Key="MyString">Hello</sys:String>
    
</ResourceDictionary>
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:sys="clr-namespace:System;assembly=mscorlib"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    
    <sys:String x:Key="MyString">Hello</sys:String>
    
</ResourceDictionary>
<Window x:Class="Test.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">
    <Grid>
        <Button Click="HandleClick" Content="{DynamicResource MyString}"/>
    </Grid>
</Window>
using System;
using System.Windows;

namespace Test
{
    public partial class MainWindow : Window
    {
        private bool _remove;
        public MainWindow()
        {
            InitializeComponent();
        }

        private void HandleClick(object sender, RoutedEventArgs e)
        {
            if (_remove)
                Application.Current.Resources.MergedDictionaries.Remove(Application.Current.Resources.MergedDictionaries[1]);
            else
                Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary() { Source = new Uri("rd2.xaml", UriKind.Relative) });

            _remove = !_remove;
        }
    }
}

Launching ClickOnce Application with parameters from a shortcut

ClickOnce is a handy technology from Microsoft that makes it easy to deploy desktop Windows applications using the web browser. IE is naturally supported out of the box, Firefox and Chrome have add-ons and apps respectively to make ClickOnce work.

When installed, ClickOnce created shortcuts for the application under user’s Start Menu (Windows 7 and 10) in the folder whose name matches the name of the company specified in the manifest of the application. But the shortcuts to ClickOnce applications are not your typical *.lnk files, they are .appref-ms files. An appref-ms file is a text file saved in UTF-16 or “UCS2 Little Endian” format. Format is important getting it wrong will make the file unrecognizable to the system.

Here’s a typical text from the appref-ms file.

https://www.application.com/MyApplication.application#MyApplication.app, Culture=neutral, PublicKeyToken=f89fd69c4bd8bc85, processorArchitecture=x86

It consists of 4 parts separated by comas:

  1. Url#Name – I noticed that the URL is not important, but the bame is – it has to match the name of the application in the manifest
  2. Culture – in the format “Culture=”
  3. PublicKeyToken – in the format “PublicKeyToken=”
  4. Target CPU – in the format “processorArchitecture=”
  5. To pass parameters to the clickonce application you must invoke the appref.ms file either from the command line with parameters. I was hoping to create a real shortcut (lnk) file with the appref.ms file as a target along with arguments, but Windows does not make it easy. It’s a lot easier to manage shortcuts to normal executable files, so that’s the approach I would recommend in this solution.

    You can add a second windows executable file to your solution. It’s job would be to generate a temporary appref.ms file on the fly and launch it with parameters using .NET’s Process.Start command. When your real app starts, make sure to copy this helper executable to user’s %appdata%\YourAppName location or ClickOnce’s ApplicationDeployment.CurrentDeployment.DataDirectory (will only be available once properly deployed). Then you can create a shortcut to the executable anywhere you like and even add command line arguments to it. Your executable then will generate the appref.ms file on the fly somewhere in temp directory, and launch it along with the arguments you specified. You can get the parts necessary to generate this file from the ApplocationIdentity object: http://msdn.microsoft.com/en-us/library/system.applicationidentity.fullname%28v=vs.110%29.aspx

    Hope this works for you.

    PS. Don’t forget to delete the dummy appref.ms file you generate after your app launches.

Microsoft mechanism to load WPF resources is broken. (looking for a better title)

I’ve been using WPF along with .NET and have been a staunch supported of the technology for years now. The pattern of separating presentation from logic gained a lot of traction. Technologies such as iOS’s cocoa touch, JavaFX and others follow same model. Even HTML5 forced all the pure presentation/layout markup out from HTML and into CSS. So the idea is sound. But what about execution.

WPF implements this pattern using XAML. XAML markup is used to design the visual this xaml is stored in the xaml file, whose name usually matches the name of the class. For example, MyWindow.xaml. During compilation, an auto-generated file marries the code that defines the visual (MyWindow.xaml.cs) and the xaml file itself (MyWindow.xaml.cs). The autogenerate file would be named MyWindow.g.i.cs and it would contain the implementation of the InitializeComponent() method that’s referenced in the main MyWindow.xaml.cs class. The contents of the method looks something like this:

[System.Diagnostics.DebuggerNonUserCodeAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute(&quot;PresentationBuildTasks&quot;, &quot;4.0.0.0&quot;)]
public void InitializeComponent() {
	if (_contentLoaded) {
		return;
	}
	_contentLoaded = true;
	System.Uri resourceLocater = new System.Uri(&quot;/MyLibrary;component/mainwindow.xaml&quot;, System.UriKind.Relative);
	
	#line 1 &quot;..\..\MainWindow.xaml&quot;
	System.Windows.Application.LoadComponent(this, resourceLocater);
	
	#line default
	#line hidden
}

So far so good right. OK, let’s talk about some of the advanced uses of .Net. Specifically support for having multiple versions of same assembly (dll) to be loaded in a single AppDomain. Such use case comes handy in a framework use case. A framework supports multiple plug-in modules, each can be developed by other developers who may rely on different versions of common libraries. I am not talking about things like log4net, I am talking about shared assemblies that define common logic and can be reused by each module. If 2 modules are loaded into a single app-domain and both depend on their ‘private’ version of the common assembly, a good framework should not force both modules to share latest version of the library, because

  • ‘latest’ does not mean backwards-compatible
  • ‘latest’ may have bugs that ‘earlier’ version did not
  • …I can go on for a while on this one. It’s 2014, dll hell is well known to software developers of the 20th century.

Now, let’s say that 2 different versions of the shared library called MyLibrary are loaded and one version is used by Module1 and another version is used by Module2. During compilation, assembly name along with the version are recorded in modules’ manifests so that when .NET encounters request to execute code defined in MyLibrary the source module will tell .Net which version of MyLibrary contains the code it wants executed. This is a basic building block of .NET and a very attractive feature.

Now, what about WPF? Well that’s not that clear is it? Look at Line #8 above: System.Uri resourceLocater = new System.Uri(“/MyLibrary;component/mainwindow.xaml”, System.UriKind.Relative). It defines what it calls a resourcelocator which the system will use to find the xaml resouce in an assembly called MyLibrary. I don’t know how WPF resolves the assembly, but if I were to write the logic for resolution, I would not have enough information to find a proper assembly MyLibrary, because I don’t know which version to load. I would have to guess. And as we know in any science guessing is bad. Computer Science is an exact… well… science. Guessing, hoping, holding fingers crossed or other techniques that do not produce determinate result will break at the worst possible moment! I can see how developers armed with inaccurate knowledge or with little experience make mistakes like this, but when Microsoft’s tools generated code that’s faulty, and make it impossible to intercept and fix, there is no other way to call it – they fucked up! How can they expect developers to trust their tooling if they, themselves, auto-generate code that sometimes breaks.

This breaks in a most bizarre way, too. MyLibrary version 2 was loading the MainWindow class, which then was running the InitializeComponent() method above, encountered the LoadComponent call, which asked to load the resource from the MyLibrary assembly without a version, and the code picked the wrong version of the MyLibrary assembly, which did not have the MyWindow class, nor the resource. What a way to blow up!

Such nonsense was reported to them too! This connect ticket marked as “Wont’t Fix” describes the problem: https://connect.microsoft.com/VisualStudio/feedback/details/668914/xaml-generated-code-uses-resource-uri-without-assembly-strong-name. StackOverflow also has references to this problem: http://stackoverflow.com/questions/1453107/how-to-force-wpf-to-use-resource-uris-that-use-assembly-strong-name-argh/6341678#6341678. Luckily one of the suggestions led me to the best solution.

The Fix


We were lucky to have our builds automated via scripts which rely on MSBuild to build the solutions. All I needed to do is to modify the script and pass the /p:AssemblyInfo=$version parameter to MSBuild. (http://stackoverflow.com/a/26689750/195275). This would tell the build system to add assembly version to the Resource reference in line 8. It now looks like the code below. Problem solved, but could have been avoided on the first place!

Shame on you, Microsoft!

[System.Diagnostics.DebuggerNonUserCodeAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute(&quot;PresentationBuildTasks&quot;, &quot;4.0.0.0&quot;)]
public void InitializeComponent() {
	if (_contentLoaded) {
		return;
	}
	_contentLoaded = true;
	System.Uri resourceLocater = new System.Uri(&quot;/MyLibrary;V1.0.23;component/mainwindow.xaml&quot;, System.UriKind.Relative);
	
	#line 1 &quot;..\..\MainWindow.xaml&quot;
	System.Windows.Application.LoadComponent(this, resourceLocater);
	
	#line default
	#line hidden
}

Am I smarter than the fifth grader?

I thought I was, until my 10 year-old brought home a math homework from her school asking her to solve a few problems that look like this:

Replace letters with digits from this list [1, 2, 3, 4, 5, 7, 8, 9]
All letters to digit mapping is unique

   CAN
x  SAY
======
 BRAND

I thought such problem would have a quick-and-easy solution, like any others in a 5th grade math, but I was stumped. I tried several tricks to see if there was a shortcut to solving these, but found nothing. The only way to solve a problem like this was to iteratively try out various number combinations to see if the result would match the constraints of the problem.

Constraints

In a problem above the constraints are as follows:

  1. 2nd digit in the 2nd number must match 2nd digit in the 1st number
  2. 2nd and 3rd digit in the 1st number must match 3rd digit and 4th digits result
  3. Result must have 5 digits
  4. None of the digits of the result except for the 3rd and 4th can be present in the first two numbers

Solving such problem.

I learned that the type of these problems are called cryptarithms or verbal arithmetic. There’s a wikipedia page dedicated to these problems as well as a laundry list of sites hosting examples and explanations to these. There is no simple solution to such problem. The solution to them is defined as having NP-Complete in its difficulty, which according to Wikipedia is

In computational complexity theory, a decision problem is NP-complete when it is both in NP and NP-hard. The set of NP-complete problems is often denoted by NP-C or NPC. The abbreviation NP refers to “nondeterministic polynomial time”.

Although any given solution to an NP-complete problem can be verified quickly (in polynomial time), there is no known efficient way to locate a solution in the first place; indeed, the most notable characteristic of NP-complete problems is that no fast solution to them is known.

My Solution

The only way to solve it was to write a program, which I did below. I figured that I have to start with a basic mapping of letters to digits and “increment” the mapping by “1” at every iteration. The difficult part was to implement the “increment” because no two letters can be mapped to the single digit, I could not completely mimic the way we increment regular numbers in base 10. Plus the universe of digits can also be constrained if the problem creator chooses to do so.

The code is written in C#, and is lazy-coded, where the inputs are hard-coded in the main method, because I own the code and can easily change the definitions. It supports multiplication and addition. It managed to solve all the problems that I found online, so I am fairly confident that it works.

Perhaps I am still smarter than the 5th grader, even if slightly.

namespace WordMultiplication
{
    class Program
    {
        static void Main(string[] args)
        {
            var digits = new int[] { 1, 2, 3, 4, 5,  7, 8, 9 }.OrderBy(n => n).ToArray();
            var word1 = "CAN";
            var word2 = "SAY";
            var result = "BRAND";

            var orderedInputLetters = word1.Union(word2).Distinct().ToArray();
            if (orderedInputLetters.Length > digits.Length)
            {
                Console.WriteLine("More letters than digits!");
                return;
            }

            var numbersToLetterMap = new SortedDictionary<int, char?>();
            var lettersToNumbersMap = new Dictionary<char, int>();

            InitialSetup(orderedInputLetters, digits, numbersToLetterMap, lettersToNumbersMap);

            bool found;
            bool couldIncrement;
            do
            {
                couldIncrement = false;
                int number1 = GetNumberForWord(word1, lettersToNumbersMap);
                int number2 = GetNumberForWord(word2, lettersToNumbersMap);

                var mathResult = number1 * number2;

                found = number1.ToString().Length == word1.Length &&
                        number2.ToString().Length == word2.Length &&
                        Verify(mathResult, result, word1, word2, new Dictionary<char, int>(lettersToNumbersMap), digits);
                if (found)
                    Console.WriteLine("{0} x {1} = {2}", number1, number2, mathResult);
                else
                    couldIncrement = TryIncrement(orderedInputLetters.Length - 1, orderedInputLetters, digits, numbersToLetterMap, lettersToNumbersMap);


            } while (!found && couldIncrement);

            if (!found)
                Console.WriteLine("Result not found");
        }

        private static bool TryIncrement(int index, char[] inputLetters, int[] digits,
            IDictionary<int, char?> numbersToLetterMap, IDictionary<char, int> lettersToNumbersMap)
        {
            if (index == -1)
                return false;

            var currLetter = inputLetters[index];
            var currDigit = lettersToNumbersMap[currLetter];

            int nextDigit;
            //If we reached the end
            if (!numbersToLetterMap.Any(kvp => kvp.Key > currDigit && !kvp.Value.HasValue))
            {
                //put current letter aside
                numbersToLetterMap[currDigit] = null;
                lettersToNumbersMap[currLetter] = -1;

                //Try to increment a digit of higher significance
                if (TryIncrement(index - 1, inputLetters, digits, numbersToLetterMap, lettersToNumbersMap))
                    //If succeeded, find the first available empty spot for the current letter
                    nextDigit = numbersToLetterMap.First(kvp => !kvp.Value.HasValue).Key;
                else
                    return false;
            }
            else
            {
                numbersToLetterMap[currDigit] = null;
                nextDigit = numbersToLetterMap.First(kvp => kvp.Key > currDigit && !kvp.Value.HasValue).Key;
            }

            numbersToLetterMap[nextDigit] = currLetter;
            lettersToNumbersMap[currLetter] = nextDigit;

            return true;
        }


        private static void InitialSetup(char[] inputLetters, int[] digits, IDictionary<int, char?> numbersToLetterMap, IDictionary<char, int> lettersToNumbersMap)
        {
            foreach (var digit in digits)
                numbersToLetterMap[digit] = null;

            foreach (var c in inputLetters)
                lettersToNumbersMap[c] = -1;

            //Place letters in the beginning of the digit line
            for (int i = 0; i < inputLetters.Length; i++)
            {
                lettersToNumbersMap[inputLetters[i]] = digits[i];
                numbersToLetterMap[digits[i]] = inputLetters[i];
            }
        }

        private static bool Verify(int mathResult, string result, string word1, string word2, IDictionary<char, int> allletters, int[] digits)
        {
            if (mathResult.ToString().Length != result.Length)
                return false;

            for (int i = 0; i < result.Length; i++)
            {
                var resultChar = result[i];
                var digitForChar = (mathResult / (int)Math.Pow(10, result.Length - i - 1) % 10);
                
                //Can result be even used?
                if (Array.IndexOf(digits, digitForChar) == -1)
                    return false;

                if (allletters.Keys.Contains(resultChar))
                {
                    if (allletters[resultChar] != digitForChar)
                        return false;
                }
                else if (allletters.Values.Contains(digitForChar))
                {
                    if (allletters.First(kvp => kvp.Value == digitForChar).Key != resultChar)
                        return false;
                }
                else
                {
                    allletters[resultChar] = digitForChar;
                }
            }

            return true;

        }

        private static int GetNumberForWord(string word, IDictionary<char, int> allletters)
        {
            var number = 0;
            for (int i = 0; i < word.Length; i++)
            {
                number += allletters[word[i]] * (int)Math.Pow(10, (word.Length) - i - 1);
            }

            return number;
        }
    }
}

WPF: The real difference between ContentControl and ContentPresenter

Most of the resources on the web specify that ContentPresenter supports a special property called ContentSource, with a default value of “Content“, which makes it easy for it to automatically set the values of these properties used to render content:

    • Content
    • ContentTemplate
    • ContentTemplateSelector
    • ContentStringFormat (3.5 sp1 +)

Basically, the property specifies the string prefix used to bind to properties of the parent. If you change the value of the ContentSource property to something else, like “Header“, the ContentPresenter’s properties would auto-bind to bind to these properties of the control you are templating using ControlTemplate:

    • Header
    • HeaderTemplate
    • HeaderTemplateSelector
    • HeaderStringFormat (3.5 sp1 +)

The web is siltent, however, about one major difference in behavior that’s important to note. 

ContentPresenter’s DataContext is automatically set to the value of its Content property, while ContentControl’s DataContext is not.

Why is it important?  In one word – bindings.  Bindings are resolved relatived to the value of the  DataContext property.  If you declare a binding on the ContentPresenter, the moment its content is set, the binding would be re-evaluated.  For example, let’s say you are building a dual-content control like this:

<UserControl x:Class="ContentPresenterVsContentControl.DualContentControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <ContentPresenter Grid.Row="0" 
                          Content="{Binding ContentOne}" 
                          ContentTemplate="{Binding ContentOneTemplate}" 
                          ContentTemplateSelector="{Binding ContentOneTemplateSelector}"/>
        <ContentPresenter Grid.Row="1" 
                          Content="{Binding ContentTwo}" 
                          ContentTemplate="{Binding ContentTwoTemplate}" 
                          ContentTemplateSelector="{Binding ContentTwoTemplateSelector}"/>
    </Grid>
</UserControl>

using System.Windows;
using System.Windows.Controls;

namespace ContentPresenterVsContentControl
{
    /// <summary>
    /// Interaction logic for DualContentControl.xaml
    /// </summary>
    public partial class DualContentControl : UserControl
    {
        public static readonly DependencyProperty ContentOneProperty = DependencyProperty.Register("ContentOne", typeof(object), typeof(DualContentControl));
        public static readonly DependencyProperty ContentOneTemplateProperty = DependencyProperty.Register("ContentOneTemplate", typeof(DataTemplate), typeof(DualContentControl));
        public static readonly DependencyProperty ContentOneTemplateSelectorProperty = DependencyProperty.Register("ContentOneTemplateSelector", typeof(DataTemplateSelector), typeof(DualContentControl));

        public static readonly DependencyProperty ContentTwoProperty = DependencyProperty.Register("ContentTwo", typeof(object), typeof(DualContentControl));
        public static readonly DependencyProperty ContentTwoTemplateProperty = DependencyProperty.Register("ContentTwoTemplate", typeof(DataTemplate), typeof(DualContentControl));
        public static readonly DependencyProperty ContentTwoTemplateSelectorProperty = DependencyProperty.Register("ContentTwoTemplateSelector", typeof(DataTemplateSelector), typeof(DualContentControl));


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

        public object ContentOne
        {
            get { return GetValue(ContentOneProperty); }
            set { SetValue(ContentOneProperty, value); }
        }

        public DataTemplate ContentOneTemplate
        {
            get { return (DataTemplate) GetValue(ContentOneTemplateProperty); }
            set { SetValue(ContentOneTemplateProperty, value); }
        }


        public DataTemplateSelector ContentOneTemplateSelector
        {
            get { return (DataTemplateSelector) GetValue(ContentOneTemplateSelectorProperty); }
            set { SetValue(ContentOneTemplateSelectorProperty, value); }
        }

        public object ContentTwo
        {
            get { return GetValue(ContentTwoProperty); }
            set { SetValue(ContentTwoProperty, value); }
        }

        public DataTemplate ContentTwoTemplate
        {
            get { return (DataTemplate)GetValue(ContentTwoTemplateProperty); }
            set { SetValue(ContentTwoTemplateProperty, value); }
        }


        public DataTemplateSelector ContentTwoTemplateSelector
        {
            get { return (DataTemplateSelector)GetValue(ContentTwoTemplateSelectorProperty); }
            set { SetValue(ContentTwoTemplateSelectorProperty, value); }
        }

    }
}

Now, let’s use it in the project:

<Window x:Class="ContentPresenterVsContentControl.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:ContentPresenterVsContentControl"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <local:DualContentControl 
            ContentOne="Hello"
            ContentTwo="World">
            
            <local:DualContentControl.ContentOneTemplate>
                <DataTemplate>
                   <Label Background="Orange" Content="{Binding}" 
                          HorizontalContentAlignment="Center" VerticalContentAlignment="Center"/>
                </DataTemplate>
            </local:DualContentControl.ContentOneTemplate>

            <local:DualContentControl.ContentTwoTemplate>
                <DataTemplate>
                    <Label Background="Green" Content="{Binding}" 
                           HorizontalContentAlignment="Center" VerticalContentAlignment="Center"/>
                </DataTemplate>
            </local:DualContentControl.ContentTwoTemplate>
        </local:DualContentControl>
    </Grid>
</Window>

You would expect something like this,image below, right?

DualContentControlWindow

Well, if you used ContentPresenters in your UserControl, you would see this, instead:

DualContentControlWindowBad

You would also see strange binding errors in the console

System.Windows.Data Error: 40 : BindingExpression path error: 'ContentTwoTemplate' 
property not found on 'object' ''String' (HashCode=-1506748533)'. 
BindingExpression:Path=ContentTwoTemplate; DataItem='String' (HashCode=-1506748533); 
target element is 'ContentPresenter' (Name=''); target property is 'ContentTemplate' (type 'DataTemplate')

System.Windows.Data Error: 40 : BindingExpression path error: 'ContentTwoTemplateSelector' 
property not found on 'object' ''String' (HashCode=-1506748533)'. BindingExpression:Path=ContentTwoTemplateSelector; DataItem='String' (HashCode=-1506748533); 
target element is 'ContentPresenter' (Name=''); target property is 'ContentTemplateSelector' (type 'DataTemplateSelector')

System.Windows.Data Error: 40 : BindingExpression path error: 'ContentOneTemplate' 
property not found on 'object' ''String' (HashCode=-694847)'. 
BindingExpression:Path=ContentOneTemplate; DataItem='String' (HashCode=-694847); 
target element is 'ContentPresenter' (Name=''); target property is 'ContentTemplate' (type 'DataTemplate')

System.Windows.Data Error: 40 : BindingExpression path error: 'ContentOneTemplateSelector' 
property not found on 'object' ''String' (HashCode=-694847)'. BindingExpression:Path=ContentOneTemplateSelector; DataItem='String' (HashCode=-694847); 
target element is 'ContentPresenter' (Name=''); target property is 'ContentTemplateSelector' (type 'DataTemplateSelector')

Notice that for some reason the ContentOneTemplate, ContentOneTemplateSelector, ContentTwoTemplate and ContentTwoTemplateSelector are bound to an object of type ‘String’. Why? Because the moment you sent the ContentProperty of the ContentPresenter its DataContext was switch to match the value of the Content property. Since the Content of the both ContentPresenters are string, “Hello” and “World” all other bindings on the ContentPresenter are now resolved against these string values!

If you replace ContentPresenters with ContentControls, the system works as expected.

<UserControl x:Class="ContentPresenterVsContentControl.DualContentControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <ContentControl Grid.Row="0" 
                          Content="{Binding ContentOne}" 
                          ContentTemplate="{Binding ContentOneTemplate}" 
                          ContentTemplateSelector="{Binding ContentOneTemplateSelector}"/>
        <ContentControl Grid.Row="1" 
                          Content="{Binding ContentTwo}" 
                          ContentTemplate="{Binding ContentTwoTemplate}" 
                          ContentTemplateSelector="{Binding ContentTwoTemplateSelector}"/>
    </Grid>
</UserControl>