Posts Tagged ‘C#’

XamlParseException – ‘The invocation of the constructor on type…’ – A common bug

June 3, 2011 7 comments

There is an exception which many people hit when first creating dependency properties.

System.Windows.Markup.XamlParseException: The invocation of the constructor on type ‘[type name here]’ that matches the specified binding constraints threw an exception.

Don’t worry – it’s very easy to fix and also very commonly made.

You first dependency property may look something like this, especially if you use the code snippet “propdp“.

        public long MyProperty
            get { return (long)GetValue(MyPropertyProperty); }
            set { SetValue(MyPropertyProperty, value); }

        public static readonly DependencyProperty MyPropertyProperty =
                new PropertyMetadata(0));

If you created your code using the code snippet it will compile just fine and the world will be a happy place. The problem lies in the constructor of the PropertyMetadata on line 12 above where you set the default value for the property.

The definition of the constructor is:

public PropertyMetadata(object defaultValue);

The type of the parameter ‘defaultValue‘ is ‘object‘, this means that your default value will be boxed as an object type.

The error comes when the default value is needed and is unboxed. If you didn’t carefully force the type of the value going in then the most sensible type will be chosen by the compiler, which may differ from the type you specified on line 10, i.e. the underlying type you wish to use for your dependency property.

Using the above as an example, the default value is set to ‘0‘ – the compiler will interpret ‘0‘ to be an Int32 and then the constructor will store that Int32 typed value as the default value in the defaultValue object.

When you come to need the default value the code that is executed behind the scenes will attempt to turn your default value back to the type specified for your dependency property, i.e.

long value = (long)defaultValue;

Much to many people’s surprise, this is not possible and will throw the exception above. Why? Well, you need to unbox back to the underlying type before you cast it to another type.

Take the following code, which you can paste into a console project to check out:

var myInt = 0;
Console.WriteLine(myInt.GetType().Name); // produces Int32

object myObj = myInt;
Console.WriteLine(myObj.GetType().Name); // produces Int32 - the underlying type

// no problem
long myLong1 = (long)myInt;

// no problem - unboxing to int and then casting to long
long myLong2 = (long)(int)myObj;

// exception - can't unbox to a different type
long myLong3 = (long)myObj;

Note that on line 1 I declared the variable using var instead of int, however we can still see that the compiler has decided that Int32 is the best way to go with this value.

So, back to the exception, and how to correct it. Simple really, make sure that the compiler is aware of the type of value that you are passing in and make sure that it matches the type you are declaring for your dependency property.

In the example above we only need to replace line 12 with the following using an explicit cast.

                new PropertyMetadata((long)0));

Alternatively we could have used a type suffixes as a shortcut.

 new PropertyMetadata(0l));

Other type suffices are:
u – uint
l – long
ul – ulong
f – float
d – double
m – decimal

If you are interested in a more depth article on boxing and unboxing and casting in general, see Eric Lippert’s excellent blog post.

WPF, MVVM and the TreeView Control using with different HierarchicalDataTemplates and DataTemplateSelector – Reloaded

June 1, 2011 1 comment

In this previous post I outlined a way to work with the TreeView control in an MVVM fashion. In order to make any sense of this post you will need to refer to the previous one.

The code, in order to keep it simple, was quite verbose.

This post is really to outline a shorter, more generic, but more complex version of the hierarchy building code as per Listing 1 below.

public class HierarchyViewModel : ViewModelBase
    public CollectionView Customers { get; private set; }
    private HierarchyItemViewModel _selectedItem;

    public HierarchyViewModel(List customers, object selectedEntity)
        var customerHierarchyItemsList = BuildHierarchyList<Customer>(customers, ((a, b) => a.Number == b.Number), selectedEntity,
            x => BuildHierarchyList<Order>(x.Orders, ((a, b) => a.Number == b.Number), selectedEntity,
                y => BuildHierarchyList<Product>(y.Products, ((a, b) => a.GetHashCode() == b.GetHashCode()), selectedEntity, null)

        this.Customers = new CollectionView(customerHierarchyItemsList);

        // select the selected item and expand it's parents
        if (_selectedItem != null)
            _selectedItem.IsSelected = true;
            HierarchyItemViewModel current = _selectedItem.Parent;

            while (current != null)
                current.IsExpanded = true;
                current = current.Parent;

    private List<HierarchyItemViewModel> BuildHierarchyList<T>(List<T> sourceList, Func<T, T, bool> isSelected, object selectedEntity, Func<T, List<HierarchyItemViewModel>> getChildren)
        List<HierarchyItemViewModel> result = new List<HierarchyItemViewModel>();

        foreach (T item in sourceList)
            // create the hierarchy item and add to the list
            var hierarchyItem = new HierarchyItemViewModel(item);

            // check if this is the selected item
            if (selectedEntity != null && selectedEntity.GetType() == typeof(T) && (isSelected.Invoke(item, (T)selectedEntity)))
                _selectedItem = hierarchyItem;

            if (getChildren != null)
                var children = getChildren.Invoke(item);
                children.ForEach(x => x.Parent = hierarchyItem);
                hierarchyItem.Children = new CollectionView(children);

        return result;

Listing 1

Final thoughts…

Comments are welcome on how to approve this further. My intention was to create a Type keyed dictionary of definitions that contained Func for finding the children and IComparers for deciding if the item is selected. This dictionary of definitions could then be passed in to the method that builds the tree which would make it even cleaner and better able to cope with changes in the hierarchy. However time prevented me from finalising this. Maybe later…

Downloads (100.70 kb)
WilberBeast.TreeView.Demo.doc (100.70 kb)
Same as the zip version, just renamed to .doc to help with downloading.

CompositeCollection Binding Problem – It’s not part of the Visual Tree

May 31, 2011 6 comments

XAML is fantastic, but occasionally unpredictable which can cause some confusion.

One instance of this is the CompositeCollection tag which is used to build up a collection of values from a number of sources.

An example of this might be as simple as adding a “Please choose…” to the top of a drop down list or more complex scenarios, such as building up a list of products from different suppliers. In most cases, when using MVVM these kind of situations can be handled in the View Model, but even so, it can still be useful to build up lists in the XAML.

One such example I hit recently was creating a context menu for filtering on a data grid. I wanted to filter based on the values in the column, but also add two sorting options at the top of the list. A simplified version of the XAML is shown in Listing 1 below.

        <MenuItem Header="Sort Ascending" />
        <MenuItem Header="Sort Descending" />
        <Separator />
        <CollectionContainer Collection="{Binding Path=FilterOptions}}" />

Listing 1 – Simplified code to show an example usage of CompositeCollection

However if you try to use code such as this you’ll very quickly hit the following binding problem.

Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=FilterOptions; DataItem=null; target element is ‘CollectionContainer’ (HashCode=23627591); target property is ‘Collection’ (type ‘IEnumerable’)

Why? Well it is all down to the fact that the data context of the XAML, specified at the top level that is inherited down through the elements actually goes down the Visual Tree. The killer is that the CompositeCollection, having no visual properties, is not part of the visual tree and therefore does not inherit the data context. The CompositeCollection has no clue what “FilterOptions” is in this example and thus throws a binding error.

Luckily we can easily solve this as all elements have access to the static resources of the XAML so we can easily create a bridging static resource that does the binding that we need and then use that within the composite collection. Listing 2 shows the static resource that we need in this instance.

    <CollectionViewSource x:Key="FilterOptionsBridge" Source="{Binding Path=Filters}" />

Listing 2 – Static resource bridging back to the data context

And finally we can adjust our CollectionContainer to point to the static resource, as in Listing 3.

<CollectionContainer Collection="{Binding Source={StaticResource FilterOptionsBridge}}" />

Listing 3 – Update to reference the static resource

As a side note, the CollectionViewSource tag needs to bind to an IEnumerable, so if you are binding to a CollectionView or one of the derived classes you’ll need to change the binding to be to the SourceCollection property as shown in Listing 4.

    <CollectionViewSource x:Key="FilterOptionsBridge" Source="{Binding Path=Filters.SourceCollection}" />

Listing 4 – Updated binding to reference the SourceCollection

Extension Methods on Generics

Sometimes when I’m coding I have the “oh wouldn’t it be nice if this was possible” moments, which turn into great moments when I find to my surprise on occasion that they are.

One such moment recently was realising that I can create extension methods against generic types – something that I had never thought about before. In hindsight of course this is possible, since a specified generic type is an actual static type.

The situation I found myself in recently was whilst coding a bit of filtering code to allow the user to set up dynamic filters. As such I needed a list of comparison operators for which I created an enumerated list as per listing 1 below.

public enum ComparisonOperator

Since most of the fields that the user could filter on would have similar comparison operators, I found myself adding in the same bunch for each filter type, i.e. a string based field would have ‘StartsWith’, ‘EndsWith’, ‘Contains’, etc.

My “oh wouldn’t it be nice if…” moment was wouldn’t it be nice if I could create an extension method to add in default sets of comparison operators to lists, and I found that I could as per the code in listing 2.

public static class ComparisonOperatorEnum
    public static IList AddNumericOperators(this IList comparisonOperatorList)
        return new List<ComparisonOperator>()

    public static IList AddStringOperators(this IList comparisonOperatorList)
        return new List<ComparisonOperator>()

    public static IList AddReferenceOperators(this IList comparisonOperatorList)
        return new List<ComparisonOperator>()

    public static IList AddBooleanOperators(this IList comparisonOperatorList)
        return new List<ComparisonOperator>()

This means that my filter definition code is now shorter and clearer, and for those of you who like such things, more fluent.

myFilteredField.ComparisonOperators = new List().AddStringOperators();

It also means that if I add in any additional comparison operators in the future, then I won’t have to hunt down any filtering code and manually add them in.

A happy C# camper.

Categories: C# Tags: , ,

IQueryable, ObjectQuery and Composing LINQ to Entity Framework Queries

March 31, 2011 1 comment

As discussed in a previous post, Entity Framework and WCF work very well together, but you need to be careful about LazyLoading being switched on automatically as this can serialise the entire object graph when querying via WCF which could have a very serious impact on performance.

With LazyLoading turned off however, the problem with WCF is how to bring back all the related data that is required. The answer is to use the ObjectQuery.Include method. In order to use this with LINQ you need to cast your IQueryableto an ObjectQuery.

This additionally allows you to compose queries from logical sections and finally include the additional extra entities that will be required in the results.

using (AdventureWorksLT2008Entities context = new AdventureWorksLT2008Entities())
    context.ContextOptions.LazyLoadingEnabled = false;

    // simple example to select some customers
    var customersQuery = from c in context.Customers
                         where c.LastName.StartsWith("B")
                         select c;

    // additional composition of the query which could be conditional
    var customersWithOrdersQuery = from c in customersQuery
                                   where c.SalesOrderHeaders.Count > 0
                                   select c;

    // include the SalesOrderHeaders in the results
    var customerOrders = (customersWithOrdersQuery as ObjectQuery)

    foreach (Customer c in customerOrders)
        Console.WriteLine("First Name: {0}, Last Name: {1}", c.FirstName, c.LastName);
        foreach (SalesOrderHeader soh in c.SalesOrderHeaders)
            Console.WriteLine("    SalesOrderID: {0}, OrderDate: {1}, TotalDue: {2}", soh.SalesOrderID, soh.OrderDate, soh.TotalDue);

Listing 1 – Example of composing queries and ObjectQuery<T>.Include

Entity Framework, LazyLoading & WCF

March 29, 2011 1 comment

Entity Framework and WCF work well together in providing a good back end service layer for a range of application types. One gotcha however that can easily catch you out without being very visible is LazyLoading.

LazyLoading defers the loading of related entities, e.g. Order for Customers, until they are accessed via a navigation property.

In the Entity Framework runtime, the default value of the LazyLoadingEnabled property in an instance of an ObjectContext is false. However, if you use the Entity Framework tools to create a new model and the corresponding generated classes, LazyLoadingEnabled is set to true in the object context’s constructor.

Thus it is possible for LazyLoading to be enabled without really explicitly requesting the feature.

There is no problem with this and LazyLoading is a great feature if you are not using WCF, however the serialisation of objects to XML before being sent over the wire causes all the navigation properties to be accessed throughout the model and thus it is very easy to accidentally serialise the whole object graph which consequently has the effect of slowing down the data access via WCF.

LazyLoading can be switched of explicitly in the ObjectContext using the code below or for Entity Framework as a whole by changing the properties of the EDMX.

public void QueryWithoutLazyLoading()
    using (AdventureWorksEntities context = new AdventureWorksEntities())
        context.ContextOptions.LazyLoadingEnabled = false;

Listing 1 – Disabling LazyLoading on the ObjectContext
In the next post I’ll look at how to work around not using LazyLoading with WCF as well as how to build up queries in manageable chunks that can be specialised based on logic.