How to reuse the Windows Workflow Foundation (WF) cool type browser editor in your components

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Introduction

Windows Workflow Foundation (WF) comes with a very cool UITypeEditor that allows selection of a type in the current project or any of its references:

Its type is System.Workflow.ComponentModel.Design.TypeBrowserEditor defined in the System.Workflow.ComponentModel.dll assembly. However, if you try to "attach" it directly to one of your component's properties, you will get an error saying "The service 'System.Workflow.ComponentModel.Compiler.ITypeProvider' must be installed for this operation to suceed. Ensure that this service is available.":

Being a general fan of the Component Model, I knew what this was about, and started working on providing the WF type browser editor with the services it needs. Here are the implementation steps I used to get it to work:

1. It was obvious that I could not use the editor as-is, so the first step was to create a custom UITypeEditor that would call the TypeBrowserEditor in WF:

   public class TypeBrowser : UITypeEditor 
   { 
       TypeBrowserEditor editor = new TypeBrowserEditor(); 
       ContextProxy flyweight;
    
       public override object EditValue(ITypeDescriptorContext context, 
                       IServiceProvider provider, object value) 
       { 
           // Use flyweight pattern to improve performance.
   
           // It's guaranteed that no more than one instance of  
   
           // this editor can ever be used at the same time. (it's modal) 
   
           if (flyweight == null) 
           { 
               flyweight = new ContextProxy(context); 
           } 
           else 
           { 
               flyweight.SetContext(context);     
           } 
    
           return editor.EditValue(flyweight, flyweight, value); 
       } 
    
       public override UITypeEditorEditStyle 
              GetEditStyle(ITypeDescriptorContext typeDescriptorContext) 
       { 
           return editor.GetEditStyle(typeDescriptorContext); 
       } 
   }

2. The code highlighted above shows that I'm passing a custom ITypeDescriptorContext to the WF editor. This custom context acts as a proxy to the real one received by the editor, and allows me to provide the additional services required by it. It provides the only service required for the editor to work, ITypeProvider:

   private class ContextProxy : ITypeDescriptorContext 
   { 
       ITypeProvider typeProvider; 
       ITypeDescriptorContext realContext; 
    
       public ContextProxy(ITypeDescriptorContext realContext) 
       { 
           this.realContext = realContext; 
       } 
    
       internal void SetContext(ITypeDescriptorContext realContext) 
       { 
           this.realContext = realContext; 
       } 
    
       #region ITypeDescriptorContext Members 
       ... 
       #endregion 
    
       #region IServiceProvider Members 
    
       public object GetService(Type serviceType) 
       { 
           if (serviceType == typeof(ITypeProvider)) 
           { 
               if (typeProvider == null)
                   typeProvider = new CustomTypeProvider(this); 
    
               return typeProvider; 
           } 
    
           return realContext.GetService(serviceType); 
       } 
    
       #endregion 
   }

3. The custom ITypeProvider instantiated and returned from the GetService call must give the browser the list of types available in the current project. For that, I took advantage of the VS-provided DynamicTypeService and ITypeDiscoveryService, as explained in my previous post:

   private class CustomTypeProvider : ITypeProvider 
   { 
       Dictionary<string, Type> availableTypes; 
    
       public CustomTypeProvider(IServiceProvider provider) 
       { 
           DynamicTypeService typeService = 
               (DynamicTypeService)provider.GetService(
                typeof(DynamicTypeService)); 
           Debug.Assert(typeService != null, 
                 "No dynamic type service registered."); 
    
           IVsHierarchy hier = VsHelper.GetCurrentHierarchy(provider); 
           Debug.Assert(hier != null, 
                 "No active hierarchy is selected."); 
    
           ITypeDiscoveryService discovery = 
                typeService.GetTypeDiscoveryService(hier); 
           Project dteProject = VsHelper.ToDteProject(hier); 
    
           availableTypes = new Dictionary<string, Type>(); 
           foreach (Type type in discovery.GetTypes(typeof(object), false))
           { 
               if (!availableTypes.ContainsKey(type.FullName)) 
               { 
                   if (type.Assembly.GetName().Name != 
                  (string)dteProject.Properties.Item("AssemblyName").Value) 
                   { 
                       availableTypes.Add(type.FullName, type); 
                   } 
                   else 
                   { 
                       availableTypes.Add(type.FullName, 
                                 new ProjectType(type)); 
                   } 
               } 
           } 
       } 
   }

   From that point on, all the ITypeProvider members use the availableTypes field for their implementation. The only caveat was that apparently the WF type browser determines that a type belongs to the current project by the presence of a null value in its Assembly property. However, the VS built-in ITypeDiscoveryService gives you fully loaded types, complete with the Assembly property and everything, even for the ones in the current project. Hence, the code above detects if the assembly of a type matches the current, and adds to the list a System.Reflection.TypeDelegator-derived class (ProjectType above) instead of the real type:

   if (type.Assembly.GetName().Name != 
         (string)dteProject.Properties.Item("AssemblyName").Value) 
   { 
       availableTypes.Add(type.FullName, type); 
   } 
   else 
   { 
       availableTypes.Add(type.FullName, new ProjectType(type)); 
   }

   The TypeDelegator class is a not-so-known type that allows a very elegant way to proxy calls to the Type it's delegating to, effectively allowing a kind of reflection interception. This interception is almost transparent, save for the creation of the type delegator with the real Type, as the delegator inherits from Type itself. My delegator is simple, and provides what the WF browser expects: a null value for the Assembly property:

   private class ProjectType : TypeDelegator 
   { 
       public ProjectType(Type delegatingType) : base(delegatingType) { } 
    
       public override Assembly Assembly 
       { 
           get { return null; } 
       } 
   }

   So the relevant members of the ITypeProvider interface take into account this trick as follows:

   #region ITypeProvider Members 
    
   public Type GetType(string name, bool throwOnError) 
   { 
       if (String.IsNullOrEmpty(name)) 
       { 
           return null; 
       } 
    
       if (availableTypes.ContainsKey(name)) 
       { 
           Type type = availableTypes[name]; 
           if (type is TypeDelegator) 
           { 
               return ((TypeDelegator)type).UnderlyingSystemType; 
           } 
           else 
           { 
               return type; 
           } 
       } 
       else 
       { 
           if (throwOnError) 
           { 
               throw new TypeLoadException(); 
           } 
           else 
           { 
               return null; 
           } 
       } 
   } 
    
   public Type GetType(string name) 
   { 
       return GetType(name, false); 
   } 
    
   public Type[] GetTypes() 
   { 
       Type[] result = new Type[availableTypes.Count]; 
       availableTypes.Values.CopyTo(result, 0); 
    
       return result; 
   }

   The GetType method is called by the WF browser when you select a type in the listview, and for setting the final return value. So, I achieve the browser requirements for display, but get a fully qualified type name (or the type itself, depending on the type of the property/value being edited) as an output.

With that code in-place, you can simply annotate your component members as follows:

public partial class MyComponent : Component 
{ 
    public MyComponent() 
    { 
        InitializeComponent(); 
    } 
 
    public MyComponent(IContainer container) 
    { 
        container.Add(this); 
 
        InitializeComponent(); 
    } 
 
    private Type someType; 
 
    [Editor(typeof(TypeBrowser), typeof(UITypeEditor))] 
    public Type DataContractType 
    { 
        get { return someType; } 
        set { someType = value; } 
    } 
}

This could also be a property on a user control, of course. With this simple attribute, you can now get the cool WF type editor for your property too:

In the above screenshot, you see:

• A simple Class Library project containing a component that launches the WF type browser (no need for a WF project!!!).
• The component we showed previously in source form, being dropped on a WinForm, and its DataContractType property having the "..." for launching the editor.
• The editor with the proper value selected (the one that was already set on the property) as well as the property in the property grid reflecting this value (serialization to code works flawlessly).

What you also see above that I have not explained yet is how to filter the dialog. In the screenshot, you see a custom label with the text "Showing types that have DataContractAttribute.", and also from all the types defined in the project or its references, only a few are shown in the treeview/listview. You can achieve this by providing custom filters that implement the ITypeFilterProvider interface. For example, the filter in action above is coded as follows:

public class DataContractFilter : ITypeFilterProvider 
{ 
    public DataContractFilter() 
    { 
    } 
 
    public DataContractFilter(IServiceProvider provider) 
    { 
        // Constructor required by the dialog. 

    } 
 
    #region ITypeFilterProvider Members 
 
    public bool CanFilterType(Type type, bool throwOnError) 
    { 
        bool isDataContract = type.IsPublic && 
             Attribute.IsDefined(type, typeof(DataContractAttribute)); 
        if (throwOnError && !isDataContract) 
        { 
            throw new ArgumentException("Type is not a data contract."); 
        } 
 
        return isDataContract; 
    } 
 
    public string FilterDescription 
    { 
        get { return "types that have DataContractAttribute"; } 
    } 
 
    #endregion 
}

Finally, you apply the filter to your property with another attribute:

[TypeFilterProvider(typeof(DataContractFilter))]
[Editor(typeof(TypeBrowser), typeof(UITypeEditor))]
public Type DataContractType

It took quite some time and reflecting, but I'm very satisfied with the result :D

Enjoy!

Related readings

• How to convert from EnvDTE.Project to IVsHierarchy and IVsProject and viceversa.
• Retrieving available types in the current project and their references (without locking).
• How to get services from the VS DTE (using System.IServiceProvider).

History

• 2007-02-12: Updated source code
• 2007-01-31: Updated for .NET 3.0 RTM.
• 2006-10-01: Initial version complete with support for filters.