If you're building custom Windows Workflow Foundation activities, then you'll likely want to write your own custom ActivityValidator-derived classes, in order to provide your own custom validation logic to ensure activities are correctly configured at design time.

Here's a very simple sample ActivityValidator class:

   /// <summary>

   /// Validator class for the MsmqReceiveActivity

   /// </summary>

   public class MsmqReceiveActivityValidator : ActivityValidator

   {

      /// <summary>

      /// Validate the receive activity

      /// </summary>

      /// <param name="manager">Validation Manager to use</param>

      /// <param name="obj">Activity instance to validate</param>

      /// <returns>Collection of validation errors</returns>

      public override ValidationErrorCollection Validate(ValidationManager manager, object obj)

      {

         ValidationErrorCollection errors = base.Validate(manager, obj);

         MsmqReceiveActivity act = (MsmqReceiveActivity)obj;

         if ( !act.IsBindingSet(MsmqReceiveActivity.QueueProperty) )

         {

            if ( act.GetValue(MsmqReceiveActivity.QueueProperty) == null )

            {

               errors.Add(ValidationError.GetNotSetValidationError("Queue"));

            }

         }

         if ( act.MessageType == null )

         {

            errors.Add(ValidationError.GetNotSetValidationError("MessageType"));

         }

         if ( !act.IsBindingSet(MsmqReceiveActivity.MessageReceivedProperty) )

         {

            errors.Add(ValidationError.GetNotSetValidationError("MessageReceived"));

         }

         return errors;

      }

   } // class MsmqReceiveActivityValidator

This validator simply does the following checks:

1. Ensures that the Queue property (which is a dependency property) of the activity is either bound to an external property, or has a direct value.
2. Ensures that the MessageType property of the activity has an actual value assigned to it.
3. Ensures that the MessageReceive property of the activity (an attached dependency property) has been explicitly bound.

The validator is then attached to the activity via the [ActivityValidator] attribute, like this:

[SRDescription("Recv_Description")]

[ToolboxItem(typeof(ActivityToolboxItem))]

[Designer(typeof(MsmqReceiveActivityDesigner), typeof(IDesigner))]

[ToolboxBitmap(typeof(MsmqReceiveActivity), "Resources.MessageQueuing.bmp")]

[ActivityValidator(typeof(MsmqReceiveActivityValidator))]

public partial class MsmqReceiveActivity

   : Activity, IEventActivity, IActivityEventListener<QueueEventArgs>