如何从XAML正确加载WF4工作流？

Question

Short version:

How do I load a WF4 workflow from XAML? Important detail: The code that loads the workflow shouldn't need to know beforehand which types are used in the workflow.

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Long version:

I am having a very hard time loading a WF4 workflow from the XAML file create by Visual Studio. My scenario is that I want to put this file into the database to be able to modify it centrally without recompiling the Workflow invoker.

I am currently using this code:

var xamlSchemaContext = new XamlSchemaContext(GetWFAssemblies());
var xmlReaderSettings = new XamlXmlReaderSettings();
xmlReaderSettings.LocalAssembly = typeof(WaitForAnySoundActivity).Assembly;
var xamlReader = ActivityXamlServices.CreateBuilderReader(
                     new XamlXmlReader(stream, xmlReaderSettings), 
                     xamlSchemaContext);

var activityBuilder = (ActivityBuilder)XamlServices.Load(xamlReader);
var activity = activityBuilder.Implementation;
var validationResult = ActivityValidationServices.Validate(activity);

This gives me a whole lot of errors, which fall into two categories:

Category 1:
Types from my assemblies are not known, although I provided the correct assemblies to the constructor of XamlSchemaContext .

ValidationError { Message = Compiler error(s) encountered processing expression "GreetingActivationResult.WrongPin". 'GreetingActivationResult' is not declared. It may be inaccessible due to its protection level. , Source = 10: VisualBasicValue, PropertyName = , IsWarning = False }

This can be solved by using the technique described here , which basically adds the assemblies and namespaces of all used types to some VisualBasicSettings instance:

var settings = new VisualBasicSettings();
settings.ImportReferences.Add(new VisualBasicImportReference
{
    Assembly = typeof(GreetingActivationResult).Assembly.GetName().Name,
    Import = typeof(GreetingActivationResult).Namespace
}); 
// ...
VisualBasic.SetSettings(activity, settings);
// ... Validate here

This works but makes the whole "dynamic loading" part of the Workflow a joke, as the code still needs to know all used namespaces.
Question 1: Is there another way to get rid of these validation errors without the need to know beforehand which namespaces and assemblies are used?

Category 2:
All my input arguments are unknown. I can see them just fine in activityBuilder.Properties but I still get validation errors saying they are unknown:

ValidationError { Message = Compiler error(s) encountered processing expression "Pin". 'Pin' is not declared. It may be inaccessible due to its protection level. , Source = 61: VisualBasicValue, PropertyName = , IsWarning = False }

No solution so far.
Question 2: How to tell WF4 to use the arguments defined in the XAML file?

Answer 1

Question 2: You can´t execute an ActivityBuilder, it´s just for design. You have to load a DynamicActivity (only through ActivityXamlServices). It should work that way (without using a special XamlSchemaContext), but you must have loaded all used assemblies in advance (placing them in the bin directory should also work, so far about Question 1, DynamicActivity might make things a little bit easier):

var dynamicActivity = ActivityXamlServices.Load(stream) as DynamicActivity;
WorkflowInvoker.Invoke(dynamicActivity);

In general, I got the impression that you´re trying to implement your own "ActivityDesigner" (like VS). I tried this myself, and it was quite hard to deal with DynamicActivity and ActivityBuilder (as DynamicActivity is not serializable but ActivityBuilder cannot be executed), so I ended up with an own activity type that internally converts one type into the other. If you want to have a look at my results, read the last sections of this article .

Answer 2

I have a project that does this - the assemblies are also stored in a database.

When it is time to instantiate a workflow instance I do the following:

1. Download the assemblies from the database to a cache location
2. Create a new AppDomain passing the assembly paths into it.
3. From the new AppDomain load each assembly - you may also need to load assemblies required by your hosting environment too.

I didn't need to mess around with VisualBasic settings - at least as far as I can see having taken a quick look in my code but I'm sure I've seen it somewhere...

In my case while I don't know the input names or types, the caller is expected to have built a request that contains the input names and values (as strings) which are then converted into the correct types via a reflection helper class.

At this point I can instantiate the workflow.

My AppDomain initialisation code looks like this:

        /// <summary>
        /// Initializes a new instance of the <see cref="OperationWorkflowManagerDomain"/> class.
        /// </summary>
        /// <param name="requestHandlerId">The request handler id.</param>
        public OperationWorkflowManagerDomain(Guid requestHandlerId)
        {
            // Cache the id and download dependent assemblies
            RequestHandlerId = requestHandlerId;
            DownloadAssemblies();

            if (!IsIsolated)
            {
                Domain = AppDomain.CurrentDomain;
                _manager = new OperationWorkflowManager(requestHandlerId);
            }
            else
            {
                // Build list of assemblies that must be loaded into the appdomain
                List<string> assembliesToLoad = new List<string>(ReferenceAssemblyPaths);
                assembliesToLoad.Add(Assembly.GetExecutingAssembly().Location);

                // Create new application domain
                // NOTE: We do not extend the configuration system
                //  each app-domain reuses the app.config for the service
                //  instance - for now...
                string appDomainName = string.Format(
                    "Aero Operations Workflow Handler {0} AppDomain",
                    requestHandlerId);
                AppDomainSetup ads =
                    new AppDomainSetup
                    {
                        AppDomainInitializer = new AppDomainInitializer(DomainInit),
                        AppDomainInitializerArguments = assembliesToLoad.ToArray(),
                        ApplicationBase = AppDomain.CurrentDomain.BaseDirectory,
                        PrivateBinPathProbe = null,
                        PrivateBinPath = PrivateBinPath,
                        ApplicationName = "Aero Operations Engine",
                        ConfigurationFile = Path.Combine(
                            AppDomain.CurrentDomain.BaseDirectory, "ZenAeroOps.exe.config")
                    };

                // TODO: Setup evidence correctly...
                Evidence evidence = AppDomain.CurrentDomain.Evidence;
                Domain = AppDomain.CreateDomain(appDomainName, evidence, ads);

                // Create app-domain variant of operation workflow manager
                // TODO: Handle lifetime leasing correctly
                _managerProxy = (OperationWorkflowManagerProxy)Domain.CreateInstanceAndUnwrap(
                    Assembly.GetExecutingAssembly().GetName().Name,
                    typeof(OperationWorkflowManagerProxy).FullName);
                _proxyLease = (ILease)_managerProxy.GetLifetimeService();
                if (_proxyLease != null)
                {
                    //_proxyLease.Register(this);
                }
            }
        }

The download assemblies code is easy enough:

        private void DownloadAssemblies()
        {
            List<string> refAssemblyPathList = new List<string>();
            using (ZenAeroOpsEntities context = new ZenAeroOpsEntities())
            {
                DbRequestHandler dbHandler = context
                    .DbRequestHandlers
                    .Include("ReferenceAssemblies")
                    .FirstOrDefault((item) => item.RequestHandlerId == RequestHandlerId);
                if (dbHandler == null)
                {
                    throw new ArgumentException(string.Format(
                        "Request handler {0} not found.", RequestHandlerId), "requestWorkflowId");
                }

                // If there are no referenced assemblies then we can host
                //  in the main app-domain
                if (dbHandler.ReferenceAssemblies.Count == 0)
                {
                    IsIsolated = false;
                    ReferenceAssemblyPaths = new string[0];
                    return;
                }

                // Create folder
                if (!Directory.Exists(PrivateBinPath))
                {
                    Directory.CreateDirectory(PrivateBinPath);
                }

                // Download assemblies as required
                foreach (DbRequestHandlerReferenceAssembly dbAssembly in dbHandler.ReferenceAssemblies)
                {
                    AssemblyName an = new AssemblyName(dbAssembly.AssemblyName);

                    // Determine the local assembly path
                    string assemblyPathName = Path.Combine(
                        PrivateBinPath,
                        string.Format("{0}.dll", an.Name));

                    // TODO: If the file exists then check it's SHA1 hash
                    if (!File.Exists(assemblyPathName))
                    {
                        // TODO: Setup security descriptor
                        using (FileStream stream = new FileStream(
                            assemblyPathName, FileMode.Create, FileAccess.Write))
                        {
                            stream.Write(dbAssembly.AssemblyPayload, 0, dbAssembly.AssemblyPayload.Length);
                        }
                    }
                    refAssemblyPathList.Add(assemblyPathName);
                }
            }

            ReferenceAssemblyPaths = refAssemblyPathList.ToArray();
            IsIsolated = true;
        }

And finally the AppDomain initialisation code:

        private static void DomainInit(string[] args)
        {
            foreach (string arg in args)
            {
                // Treat each string as an assembly to load
                AssemblyName an = AssemblyName.GetAssemblyName(arg);
                AppDomain.CurrentDomain.Load(an);
            }
        }

Your proxy class needs to implement MarshalByRefObject and serves as your communication link between your app and the new appdomain.

I find that I am able to load workflows and get the root activity instance without any problem.

EDIT 29/07/12 **

Even if you only store the XAML in the database you will need to track the referenced assemblies. Either your list of referenced assemblies will tracked in an additional table by name or you will have to upload (and obviously support download) the assemblies referenced by the workflow.

Then you may simply enumerate all the reference assemblies and add ALL namespaces from ALL public types to the VisualBasicSettings object - like this...

            VisualBasicSettings vbs =
                VisualBasic.GetSettings(root) ?? new VisualBasicSettings();

            var namespaces = (from type in assembly.GetTypes()
                              select type.Namespace).Distinct();
            var fullName = assembly.FullName;
            foreach (var name in namespaces)
            {
                var import = new VisualBasicImportReference()
                {
                    Assembly = fullName,
                    Import = name
                };
                vbs.ImportReferences.Add(import);
            }
            VisualBasic.SetSettings(root, vbs);

Finally don't forget to add namespaces from the environment assemblies - I add namespaces from the following assemblies:

• mscorlib
  
• System
  
• System.Activities
  
• System.Core
  
• System.Xml
  

So in summary:
1. Track the assembly referenced by the user's workflow (since you will be rehosting the workflow designer this will be trivial)
2. Build a list of assemblies from which namespaces will be imported - this will be a union of the default environment assemblies and the user referenced assemblies.
3. Update the VisualBasicSettings with the namespaces and reapply to the root activity.

You will need to do this in the project that executes workflow instances and in the project that rehosts the workflow designer.

Answer 3

One system that I know which does the same job that you are trying to do is the Team Foundation 2010's build system. When you execute a custom build workflow on a controller, you need to point the build controller to a path in TFS where you keep your custom assemblies. The controller then recursively loads up all the assemblies from that location as it starts processing the workflow.

You mentioned that you need to keep the file in a database. Can you not also store the location or meta data information about the required assemblies in the same database and use Reflection to load them recursively before you invoke your workflow?

You can then selectively add/remove assemblies from this path without having to alter the code that dynamically load assemblies using the

var settings = new VisualBasicSettings();
settings.ImportReferences.Add(new VisualBasicImportReference
{
    Assembly = typeof(GreetingActivationResult).Assembly.GetName().Name,
    Import = typeof(GreetingActivationResult).Namespace
}); 
// ...
VisualBasic.SetSettings(activity, settings);
// ... Validate here

approach.

Answer 4

This is the way how I load xaml embeded resource (default workflow) to a Workflow Designer:

//UCM.WFDesigner is my assembly name, 
//Resources.Flows is the folder name, 
//and DefaultFlow.xaml is the xaml name. 
 private const string ConstDefaultFlowFullName = @"UCM.WFDesigner.Resources.Flows.DefaultFlow.xaml";
      private void CreateNewWorkflow(object param)
    {

        //loading default activity embeded resource
        using (var stream = Assembly.GetExecutingAssembly().GetManifestResourceStream(ConstDefaultFlowFullName))
        {
            StreamReader sReader = new StreamReader(stream);
            string content = sReader.ReadToEnd();
            //createion ActivityBuilder from string
            ActivityBuilder activityBuilder = XamlServices.Load( ActivityXamlServices
                .CreateBuilderReader(new XamlXmlReader(new StringReader(content)))) as ActivityBuilder;
            //loading new ActivityBuilder to Workflow Designer
            _workflowDesigner.Load(activityBuilder);
            OnPropertyChanged("View");
        }
    }