Setting approvers in the Approval workflow using SPD workflow action

Question

I need to build a document approval workflow for a sharepoint document library. The workflow needs to be exactly like the OOTB approval workflow with the exception that the Approvers depend on a metadata field in the document. for Example, HR documents needs to be approved by approver1 but finance documents need to be approved by approver2.

I made a copy of the OOTB approval workflow and want to customize it by adding an action to set the value of the Approvers Parameter as shown here (In the real workflow i will add a condition to set the actual value of the parameter but for now I just need to make sure this approach is working).

I set up the value of the Approvers Parameter as:

<d:Approvers xmlns:d="http://schemas.microsoft.com/office/infopath/2009/WSSList/dataFields"
       xmlns:pc="http://schemas.microsoft.com/office/infopath/2007/PartnerControls">
  <d:Assignment>
    <d:Assignee>      
      <pc:Person>
        <pc:DisplayName>Approver1</pc:DisplayName>
        <pc:AccountId>DEMO\approver1</pc:AccountId>
        <pc:AccountType>User</pc:AccountType>
      </pc:Person>
    </d:Assignee>
    <d:Stage xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" />
    <d:AssignmentType>Serial</d:AssignmentType>
  </d:Assignment>
</d:Approvers>

I keep getting the following error in the SP Logs:

    System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> 
  System.NullReferenceException: Object reference not set to an instance of an object.     
  at Microsoft.Office.Workflow.Actions.StageContainer.UpdateFromString(SPWeb web, SPItemKey itemId, String assignmentString)     
  at Microsoft.Office.Workflow.Actions.OfficeTask.get_ParticipantList()     --- End of inner exception stack trace ---     
  at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)     
  at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)     
  at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)     
  at System.Workflow.ComponentModel.ActivityBindPropertyInfo.GetValue(Object obj, BindingFlags invokeAttr, Binder binder, Object[] index, CultureInfo culture)     
  at System.Reflection.PropertyInfo.GetValue(Object obj, Object[] index)     
  at System.Workflow.ComponentModel.MemberBind.<>c__DisplayClass1.<GetValue>b__0(Object sender, PathMemberInfoEventArgs eventArgs)     
  at System.Workflow.ComponentModel.PathWalker.TryWalkPropertyPath(Type rootType, String path)     
  at System.Workflow.ComponentModel.MemberBind.GetValue(MemberInfo memberInfo, Object dataContext, String path)     
  at System.Workflow.ComponentModel.ActivityBind.GetMemberValue(Object dataSourceObject, MemberInfo memberInfo, String path, Type targetType)     
  at System.Workflow.ComponentModel.ActivityBind.InternalGetRuntimeValue(Activity activity, Type targetType)     
  at System.Workflow.ComponentModel.DependencyObject.GetBoundValue(ActivityBind bind, Type targetType)     
  at System.Workflow.ComponentModel.DependencyObject.GetValueCommon(DependencyProperty dependencyProperty, PropertyMetadata metadata)     
  at Microsoft.SharePoint.WorkflowActions.CoerceActivity.get_Value()     
  at Microsoft.SharePoint.WorkflowActions.CoerceActivity.Coerce()     
  at Microsoft.SharePoint.WorkflowActions.CoerceActivity.Execute(ActivityExecutionContext provider)     
  at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(T activity, ActivityExecutionContext executionContext)     
  at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(Activity activity, ActivityExecutionContext executionContext)     
  at System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime)     
  at System.Workflow.Runtime.Scheduler.Run()    49157206-76a1-4fe0-8b74-0567086a80ff

Any idea what might be wrong?

Environment: Windows 2008 R2,SP2010 Enterprise edition.

Answer 1

I configured a dynamic approver by changing the "with [these users]" settings into looking up the data from the current item, as shown in step 11 here: http://roykimsharepoint.wordpress.com/2011/02/15/sharepoint-designer-walkthrough-copy-modify-publishing-workflow/

This blogpost suggests that you need to wrap the assignement XML inside an "Assignments" element when used inside a workflow: http://blog.symprogress.com/2011/10/sharepoint-2010-dynamic-approvers-for-oob-approval-workflow-part-2/

Answer 2

If you set any Person or group type field like the Parameters:Approvers above make sure you set 'Return field as' something that can identify your users unambiguously. For example login name does, full name doesn't.