PreValidateDeleteContact.cs

protected void ExecutePreValidateContactDelete(LocalPluginContext localContext)
{
    if (localContext == null)
    {
        throw new ArgumentNullException("localContext");
    }

    _service = localContext.OrganizationService;

    EntityReference contact = (EntityReference)localContext.PluginExecutionContext.InputParameters["Target"];
    
    if (HasOpenTasks(contact.Id))
    {
        throw new InvalidPluginExecutionException("This Contact has open Tasks associated with it.\n\nPlease resolve all outstanding tasks before deleting.");
    }
}

private bool HasOpenTasks(Guid contactId)
{
    var tasks = GetRelatedTasks(contactId);
    return tasks.Any();
}

private IEnumerable<Entity> GetRelatedTasks(Guid contactId)
{
    QueryExpression query = new QueryExpression()
    {
        EntityName = "task",
        ColumnSet = new ColumnSet(true),
    };

    const int STATE_OPEN = 0;
    query.Criteria.AddCondition(new ConditionExpression("regardingobjectid", ConditionOperator.Equal, contactId));
    query.Criteria.AddCondition(new ConditionExpression("statecode", ConditionOperator.Equal, STATE_OPEN));

    return _service.RetrieveMultiple(query).Entities.ToList<Entity>();
}