Invoking a XOML workflow from a workflow

The built-in InvokeWorkflow activity requires a compiled workflow type which isn't too useful if you're trying to execute a XOML-based workflow. Initially I thought I could compile the XOML workflow and then use the InvokeWorkflow activity. The problem with this approach is that the InvokeWorkflow's TargetWorkflow property can't be set at run-time.

The solution is pretty straightforward, compile the workflow then use the IStartWorkflow service to start the workflow (which is what InvokeWorkflow does internally). But there are still a couple of issues with this.

First, there is no way to pass parameters to a XOML workflow, since there is no way to add top-level properties. You could workaround this by sub-classing the SequentialWorkflowActivity and adding properties for the parameters you want to pass.

The second problem is if you want the child workflow to finish before continuing with the parent workflow. rather than starting the child workflow asynchronously. I've seen examples of how to do this by the child workflow signalling when it's complete but I've not seen this done in a generic way that doesn't require the child workflow to be modified.

Anyway, here's my code which might be useful.

using System;
using System.IO;
using System.Reflection;
using System.Workflow.ComponentModel;
using System.Workflow.ComponentModel.Compiler;
using System.Xml;

namespace WFBuild.Activities
{
  public class InvokeXomlWorkflowActivity: Activity
  {
    public InvokeXomlWorkflowActivity()
    {
    }

    private string xomlFile;
    public string XomlFile
    {
      get { return xomlFile; }
      set { xomlFile = value; }
    }

    protected override ActivityExecutionStatus Execute(ActivityExecutionContext executionContext)
    {
      string className = Path.GetFileNameWithoutExtension(xomlFile);

      // compile workflow
      WorkflowCompiler compiler = new WorkflowCompiler();
      WorkflowCompilerParameters wfParams = new WorkflowCompilerParameters();
      wfParams.LibraryPaths.Add(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location));
      wfParams.ReferencedAssemblies.Add("WFBuild.Activities.dll");
      wfParams.OutputAssembly = string.Format("{0}.dll", className);

      // add x:Class attribute
      string tempFileName = Path.GetTempPath() + "temp.xoml";
      XmlDocument doc = new XmlDocument();
      doc.Load(xomlFile);
      const string namespaceUri = "http://schemas.microsoft.com/winfx/2006/xaml";
      if (doc.DocumentElement.GetAttribute("Class", namespaceUri) == "")
      {
        doc.DocumentElement.SetAttribute("Class", namespaceUri, className);
      }
      doc.Save(tempFileName);

      WorkflowCompilerResults results = compiler.Compile(wfParams, tempFileName);

      if (results.Errors.HasErrors)
      {
        string errors = "";
        for (int i = 0; i < results.Errors.Count; i++)
        {
          errors += results.Errors[i].ErrorText + "\n";
        }
        throw new Exception(errors);
      }
      
      Type[] types = results.CompiledAssembly.GetTypes();
      Type workflow = types[0];

      IStartWorkflow service = executionContext.GetService(typeof(IStartWorkflow)) as IStartWorkflow;
      if (service == null)
      {
        throw new InvalidOperationException("No IStartWorkflow service available");
      }
      Guid guid = service.StartWorkflow(workflow, null);
      if (guid == Guid.Empty)
      {
        throw new InvalidOperationException("Failed to start the workflow");
      }
      return ActivityExecutionStatus.Closed;
    }
  }
}