The WorkflowInvoker is a static class to create and invoke a WF Workflow within the WCF Service. In order to use this class, the WorkflowRuntime core must be hosted in the appDomain. The WorkflowInvoker namespace includes WFServiceHostExtension class allowing to attach and configure the workflow runtime services:  

using (System.ServiceModel.ServiceHost host = new System.ServiceModel.ServiceHost(typeof(Pinging)))
{
  // Service extension for WF 
  WFServiceHostExtension extension = new WFServiceHostExtension("WorkflowRuntimeConfig");

  // Add the Extension to the ServiceHost collection
  host.Extensions.Add(extension);

  host.Open();

  Console.WriteLine("Press any key to stop server...");
  Console.ReadLine();
  host.Close();
}

Using the WorkflowInvoker class is straightforward like is shown in the following examples:

public string Ping(Request request)
{
  // Step 1: create workflow invoker
  Type type = typeof(WorkflowTest);
  Guid id = Guid.NewGuid();
  HybridDictionary args = null;
  Invoker invoker = WorkflowInvoker.Create(type, args, id);

  // Step 2: pass request object to the workflow
  invoker.RaiseRequestEvent(request);

  // Step 3: wait for response from workflow
  invoker.WaitForResponse(45);

  // Step 4: response
  string response = invoker.GetResponse();

  return response;
}

public string Ping2(Request request)
{
  return WorkflowInvoker.RequestResponse(typeof(WorkflowTest), request);
}

 The workflow services can be added to the WorkflowRuntime core programmatically or using a config file: 

<configSections>
  <section name="WorkflowRuntimeConfig" type="System.Workflow.Runtime.Configuration.WorkflowRuntimeSection, System.Workflow.Runtime, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
  <section name ="LocalServicesConfig" type ="System.Workflow.Activities.ExternalDataExchangeServiceSection, System.Workflow.Activities, Version=3.0.00000.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
</configSections>

<WorkflowRuntimeConfig >
  <Services>
    <add type="System.Workflow.Activities.ExternalDataExchangeService, System.Workflow.Activities, Version=3.0.00000.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" 
         ConfigurationSection="LocalServicesConfig" />
    <!--<add type="System.Workflow.Runtime.Hosting.ManualWorkflowSchedulerService, System.Workflow.Runtime, Version=3.0.00000.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>-->
  </Services>
</WorkflowRuntimeConfig>

<LocalServicesConfig>
  <Services>
    <!--<add type="RKiss.WorkflowInvoker.InvokerLocalService, WorkflowInvoker, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />-->
  </Services> 
</LocalServicesConfig>