troubles with a custom SPD activity in sandboxed solution

Question

I want to build a custom activity for SPD 2010 that can extract the full url of a workflow task.

1. I did not found something like this OOB. Did I missed something ?

2. I wrote the following code :

   public Hashtable GetWorkflowTaskUrl(SPUserCodeWorkflowContext context, int taskItemId)
   {
       System.Diagnostics.Debugger.Break();
       var result = new Hashtable();
       result["Except"] = string.Empty;
   
       try
       {
           using (var site = new SPSite(context.SiteUrl))
           {
               using (var web = site.OpenWeb(context.WebUrl))
               {
                   var list = web.Lists[context.ListId];
                   var item = list.GetItemById(context.ItemId);
                   var wfInst = item.Workflows[context.WorkflowInstanceId];
                   var taskItem =  wfInst.TaskList.GetItemById(taskItemId);
                   result["EncodedAbsUrl"] = taskItem[taskItem.Fields.GetFieldByInternalName("EncodedAbsUrl").Id];
               }
           }
       }
       catch (Exception exc)
       {
           result["Except"] = exc.ToString();
       }
       return result;
   }
   

The code seems to work, as I can step into it. However, I'm struggling with the action.xml file :

<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
    <WorkflowActions>
        <Action Name="Extraire l'url de la tâche"
                Category="Tâches"
                Assembly="$SharePoint.Project.AssemblyFullName$"
                ClassName="myproject.ListItemUrlResolver"
                FunctionName="GetWorkflowTaskUrl"
                AppliesTo="all"
                UsesCurrentItem="true"
                SandboxedFunction="true">
            <RuleDesigner Sentence="Recherche l'url de la tâche d'ID %1 et la stocke dans %2">
                <FieldBind Field="taskItemId" 
                           Text="Task Item ID"
                           DesignerType="Default"
                           Id="1"
                           />
                <FieldBind Field="EncodedAbsUrl" 
                           Text="Item url"
                           DesignerType="ParameterNames"
                           Id="2"
                           />
            </RuleDesigner>
            <Parameters>
                <Parameter Name="__Context"
                           Type="Microsoft.SharePoint.WorkflowActions.WorkflowContext, Microsoft.SharePoint.WorkflowActions"
                           Direction="In"
                           DesignerType="Hide"/>
                <Parameter Name="taskItemId"
                           Type="System.Int32, mscorlib"
                           Direction="In"

                           Description="Id de l'élément de liste"
                           />
                <Parameter Name="EncodedAbsUrl"
                           Type="System.String, mscorlib"
                           Direction="Out"
                           DesignerType="ParameterNames"
                           Description="Url absolu de l'élément"/>
            </Parameters>
        </Action>
    </WorkflowActions>
</Elements>

Especially this :

<FieldBind Field="taskItemId" 
                           Text="Task Item ID"
                           DesignerType="Default"
                           Id="1"
                           />

I didn't find what I have to put in the DesignerType attribute :

• If I put "ChooseListItem", I can't choose my current task
• If I put "Default", I get an error telling me the default is not in the enumeration
• If I put nothnig, same error

What I simply want, is to be able to set the parameter to the current task id within a task process;

thx in advance