Markup Extension for Generic Classes

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Introduction

Declaring generic types in XML is something which is sadly lacking from the current incarnation of WPF. Although they are fully supported[^] in XML 2009[^], you cannot use these features in markup-compiled XAML or BAML.

Background

In .NET 3.5, it was relatively simple to create a markup extension for generic types - for example, this implementation on the MSDN forums[^] worked perfectly.

Unfortunately, in .NET 4.0, the IXamlTypeResolver service was updated to reject type names containing "`" - the separator between the type name and type argument count for generic types.

Mayur Shah's tip[^] avoids this by side-stepping the XAML type resolver. Whilst this works at runtime, it will fail at design-time. It also makes the type declaration unnecessarily long, since you can't use XML namespaces.

The Solution

The core of the solution involves using two services: IXamlNamespaceResolver and IXamlSchemaContextProvider. This is more complicated than the .NET 3.5 solution, but achieves the same result:

var resolver = GetRequiredService<IXamlNamespaceResolver>(serviceProvider);
var schema = GetRequiredService<IXamlSchemaContextProvider>(serviceProvider);

var xamlNs = resolver.GetNamespace(namespaceName);
string genericTypeName = string.Format(CultureInfo.InvariantCulture, 
                "{0}`{1:D}", typeName, typeArguments.Length);

var xamlTypeName = new XamlTypeName(xamlNs, genericTypeName);
Type genericType = schema.SchemaContext.GetXamlType(xamlTypeName).UnderlyingType;

Type finalType = genericType.MakeGenericType(typeArguments);

Error-handling omitted for brevity.

Using the Code

The attached converter can be used in several ways:

<!-- Inline: -->
<SomeTag Type="{nbs:GenericType ge:Queue, sys:String}"/>
<SomeTag Type="{nbs:GenericType ge:SortedList, sys:String, sys:Int32}"/>

<!-- Inline, using a static resource for the type arguments: -->
<SomeTag Type="{nbs:GenericType BaseTypeName=scg:Dictionary, 
TypeArguments={StaticResource types}}"/>

<!-- Inline, using a string for the type arguments: -->
<SomeTag Type="{nbs:GenericType BaseTypeName=scg:KeyValuePair, 
TypeArguments='sys:String, sys:Int32'}"/>

<!-- Expanded, using a static resource for the type arguments: -->
<SomeTag>
    <SomeTag.Type>
        <nbs:GenericType BaseTypeName="ge:SortedList" 
        TypeArguments="{StaticResource types}"/>
    </SomeTag.Type>
</SomeTag>

<!-- Expanded, specifying the type arguments as child elements: -->
<SomeTag>
    <SomeTag.Type>
        <nbs:GenericType BaseTypeName="ge:SortedDictionary">
            <x:Type TypeName="sys:String"/>

            <!-- Even nested generic types! -->
            <nbs:GenericType BaseTypeName="scg:List">
                <x:Type TypeName="sys:Int32"/>
            </nbs:GenericType>
        </nbs:GenericType>
    </SomeTag.Type>
</SomeTag>

Points of Interest

The attached class library is decorated with the XmlnsPrefix and XmlnsDefinition attributes. This makes it easier to use the namespace in a XAML document - instead of:

xmlns:nbs="clr-namespace:XamlGenericTypeExtension;assembly=XamlGenericTypeExtension"

we can simply use:

xmlns:nbs="urn:nbs-wpf-generic"

This has the added advantage that multiple namespaces can be mapped to the same XML namespace.

History

• 6^th February, 2015 - Initial version

Further Reading

• XAML 2009 Features: Generics Support[^]
• UserVoice suggestion to support XAML 2009 in Visual Studio and compiled markup[^]