Question

A resource extension I’ve been using for a few years now stopped working at design time in a new .Net 4 project with the following error:

Markup extension ‘StaticResourceExtension’ requires ‘IXamlSchemaContextProvider’ be implemented in the IServiceProvider for ProvideValue.

The relevant method from the extension is the following:

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        Style resultStyle = new Style();
        foreach (string currentResourceKey in resourceKeys)
        {
            object key = currentResourceKey;
            if (currentResourceKey == ".")
            {
                IProvideValueTarget service = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
                key = service.TargetObject.GetType();
            }
            Style currentStyle = new StaticResourceExtension(key).ProvideValue(serviceProvider) as Style;
            if (currentStyle == null)
                throw new InvalidOperationException("Could not find style with resource key " + currentResourceKey + ".");
            resultStyle.Merge(currentStyle);
        }
        return resultStyle;
    }

Presumably, the compiler is giving the error because when I call currentStyle = new StaticResourceExtension(key).ProvideValue(serviceProvider);, the serviceProvider I’m passing along is missing IXamlSchemaContextProvider information. Don’t know where it would come from though, I don’t even know how the service provider for the markup extension gets set in the first place, I just use it like this:

<Style x:Key="ReadOnlyTextCell" TargetType="{x:Type TextBlock}" BasedOn="{util:MultiStyle ReadOnlyCell TextCell}"/>

---

The full code for the extension is here:

using System;
using System.Windows;
using System.Windows.Markup;

/* MultiStyleExtension - used to merge multiple existing styles.
 *  
 * Example:
    <Window xmlns:local="clr-namespace:FlagstoneRe.WPF.Utilities.UI">
        <Window.Resources>
            <Style x:Key="ButtonStyle" TargetType="Button">
                <Setter Property="Width" Value="120" />
                <Setter Property="Height" Value="25" />
                <Setter Property="FontSize" Value="12" />
            </Style>
            <Style x:Key="GreenButtonStyle" TargetType="Button">
                <Setter Property="Foreground" Value="Green" />
            </Style>
            <Style x:Key="RedButtonStyle" TargetType="Button">
                <Setter Property="Foreground" Value="Red" />
            </Style>
            <Style x:Key="BoldButtonStyle" TargetType="Button">
                <Setter Property="FontWeight" Value="Bold" />
            </Style>
        </Window.Resources>

        <Button Style="{local:MultiStyle ButtonStyle GreenButtonStyle}" Content="Green Button" />
        <Button Style="{local:MultiStyle ButtonStyle RedButtonStyle}" Content="Red Button" />
        <Button Style="{local:MultiStyle ButtonStyle GreenButtonStyle BoldButtonStyle}" Content="green, bold button" />
        <Button Style="{local:MultiStyle ButtonStyle RedButtonStyle BoldButtonStyle}" Content="red, bold button" />

 * Notice how the syntax is just like using multiple CSS classes.
 * The current default style for a type can be merged using the "." syntax:

        <Button Style="{local:MultiStyle . GreenButtonStyle BoldButtonStyle}" Content="Bold Green Button" />

 */

namespace FlagstoneRe.WPF.Utilities.UI
{
    [MarkupExtensionReturnType(typeof(Style))]
    public class MultiStyleExtension : MarkupExtension
    {
        private string[] resourceKeys;

        /// <summary>
        /// Public constructor.
        /// </summary>
        /// <param name="inputResourceKeys">The constructor input should be a string consisting of one or more style names separated by spaces.</param>
        public MultiStyleExtension(string inputResourceKeys)
        {
            if (inputResourceKeys == null)
                throw new ArgumentNullException("inputResourceKeys");
            this.resourceKeys = inputResourceKeys.Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
            if (this.resourceKeys.Length == 0)
                throw new ArgumentException("No input resource keys specified.");
        }

        /// <summary>
        /// Returns a style that merges all styles with the keys specified in the constructor.
        /// </summary>
        /// <param name="serviceProvider">The service provider for this markup extension.</param>
        /// <returns>A style that merges all styles with the keys specified in the constructor.</returns>
        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            Style resultStyle = new Style();
            foreach (string currentResourceKey in resourceKeys)
            {
                object key = currentResourceKey;
                if (currentResourceKey == ".")
                {
                    IProvideValueTarget service = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
                    key = service.TargetObject.GetType();
                }
                Style currentStyle = new StaticResourceExtension(key).ProvideValue(serviceProvider) as Style;
                if (currentStyle == null)
                    throw new InvalidOperationException("Could not find style with resource key " + currentResourceKey + ".");
                resultStyle.Merge(currentStyle);
            }
            return resultStyle;
        }
    }

    public static class MultiStyleMethods
    {
        /// <summary>
        /// Merges the two styles passed as parameters. The first style will be modified to include any 
        /// information present in the second. If there are collisions, the second style takes priority.
        /// </summary>
        /// <param name="style1">First style to merge, which will be modified to include information from the second one.</param>
        /// <param name="style2">Second style to merge.</param>
        public static void Merge(this Style style1, Style style2)
        {
            if(style1 == null)
                throw new ArgumentNullException("style1");
            if(style2 == null)
                throw new ArgumentNullException("style2");
            if(style1.TargetType.IsAssignableFrom(style2.TargetType))
                style1.TargetType = style2.TargetType;
            if(style2.BasedOn != null)
                Merge(style1, style2.BasedOn);
            foreach(SetterBase currentSetter in style2.Setters)
                style1.Setters.Add(currentSetter);
            foreach(TriggerBase currentTrigger in style2.Triggers)
                style1.Triggers.Add(currentTrigger);
            // This code is only needed when using DynamicResources.
            foreach(object key in style2.Resources.Keys)
                style1.Resources[key] = style2.Resources[key];
        }
    }
}

Answer 1

I’m running into the same issue. Some additional information:

At runtime, the IServiceProvider value is of type “MS.Internal.Xaml.ServiceProviderContext”.

In the Visual Studio Xaml Designer, the IServiceProvider value is of type “Microsoft.Expression.DesignModel.Core.InstanceBuilderOperations.InstanceBuilderServiceProvider”.

Clearly, VS2010 is using classes from Expression Blend to provide better design-time behavior than VS2008 had – but at a price, because the Expression Blend classes don’t have all the same information that the actual run-time system does.

MORE INFO: I tried inserting my own class that implements both IServiceProvider and IXamlSchemaContextProvider. Some calls get passed to the original IServiceProvider, and I provide a dummy (empty) XamlSchemaContext when requested. But I still get the same error.

Something inside WPF is encapsulating my IServiceProvider with another IServiceProvider type – but one that fails to implement IXamlSchemaContextProvider. I have no further ideas on how to resolve this.