R – Workflow Dependency Property

I finally managed to achieve this by attaching an ActivityToolboxItem to the Activity, and overriding a method in it that creates the instance shown in the designer. I used an ActivityBind object to bind the dependencyproperty to the workflow's property. To get the instance of the workflow, I just searched for an ancestor to my activity by calling act.Parent until the activity had no parent (and thus was the StateMachineWorkflowActivity itself)

First of all the True/False example isn't great, in that case use a bool type.

For a multi-value item why not use an Enum:-

 public enum ItemEnum
 {
    First,
    Second,
    Third
 }

Now in your Activity:-

 public static DependencyProperty TestProperty = DependencyProperty.Register("Test",  
   typeof(ItemEnum), typeof(TestActivity), new PropertyMetadata(ItemEnum.First));

[Description("Select Item value")]
[Category("Settings")]
[DefaultValue(ItemEnum.First)]
public ItemEnum Type
{
  get
  {
    return (ItemEnum)GetValue(TestActivity.TestProperty);
  }
  set
  {
    SetValue(TestActivity.TestProperty, value);
  }
}

Note the simplification of the Attributes on the property. In particular Browseable being true and DesignerSerializationVisiblity being Visible are defaults so remove them. Also the property grid is easier for the "user" to use if the DefaultValue is defined. Note also dropped the "Attribute" suffix, makes it much more straightforward to read.