Lookup fields in custom activity and [%_x005f_String0%]

If you do a custom activity, configure it to use Lookup fields on field configured as DesignerType="StringBuilder" and instead of field value you get [%_x005f_String0%] you need to process this value with this function: public static string ProcessStringField ( string str, Activity activity, WorkflowContext context ) Here is a code sample:

//... 
public static DependencyProperty __ContextProperty = 
    DependencyProperty.Register("__Context", typeof(WorkflowContext), typeof(EmailActivity)); 
//... 
[ValidationOption(ValidationOption.Optional)]public WorkflowContext __Context 
{ 
    get { return (WorkflowContext) base.GetValue(__ContextProperty); } 
    set { base.SetValue(__ContextProperty, value); }
} 
//... 
protected override ActivityExecutionStatus Execute(ActivityExecutionContext provider) 
{ 
    Activity parent = provider.Activity; 
    while (parent.Parent != null) 
    { 
        parent = parent.Parent; 
    } 
    
    string returnValue = Helper.ProcessStringField(stringToProcess, parent, this.__Context)); 
}

It seems that if you don't have WorkflowContext you can pass null

string returnValue = Helper.ProcessStringField(stringToProcess, parent, this.__Context));