In order to add a new activity to our SharePoint Designer, first  we need to begin a WorkFlow project  in Visual Studio 2005,  concretely, a Workflow Activity Library (Workflow activity library).

From  this point we would create our activity; for this example I am going to create an activity that will send a text to the event viewer.

   1:  public partial class LogEventViewer : Activity

   2:  {

   3:     

   4:      public static DependencyProperty TextLogProperty =

   5:          DependencyProperty.Register("TextLog", typeof(string), typeof(LogEventViewer));

   6:   

   7:      public LogEventViewer()

   8:      {

   9:          InitializeComponent();

  10:      }

  11:   

  12:      /// <summary>

  13:      /// Valor que figurará en el visor de eventos

  14:      /// </summary>

  15:      /// <value>Texto</value>

  16:      [Description("Texto que saldrá en el visor de eventos")]

  17:      [Category("User")]

  18:      [Browsable(true)]

  19:      [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]

  20:      public string TextLog

  21:      {

  22:          get { return (string) GetValue(TextLogProperty); }

  23:          set { SetValue(TextLogProperty, value); }

  24:      }

  25:   

  26:      /// <summary>

  27:      /// Ejecución de la actividad

  28:      /// </summary>

  29:      /// <param name="provider">Contexto de ejecución de la actividad</param>

  30:      /// <returns></returns>

  31:      protected override ActivityExecutionStatus Execute(ActivityExecutionContext provider)

  32:      {

  33:          EventLog eventLog = new EventLog("Workflow");

  34:   

  35:          eventLog.Source = "SharePoint Workflow";

  36:   

  37:          try

  38:          {

  39:              eventLog.WriteEntry(TextLog, EventLogEntryType.Information);

  40:          }

  41:          finally

  42:          {

  43:              eventLog.Dispose();

  44:          }

  45:   

  46:          return ActivityExecutionStatus.Closed;

  47:      }

  48:  }

First,  we  declare a dependent property of the workflow called TextLogProperty,  which we will use to pass the text that we wish to  show in the event viewer.

The internal property of the activity will be TextLog; this internal property obtains and establishes the value from the dependent property of the workflow.

Then we  define the Execute method, that  will be the  method in charge to  visualize the message in the event viewer.

A good practice is that the activities include a Validador, that I have omitted in the example but that is highly  recommended although  not obligatory, or if we want to use the activity inside of the visual studio workflow designer.

Once we have made the compilation, we can test it first (another good practice) creating an application to host the workflow and checking the activity.

Finally, after we are sure that our activity works correctly, we must install it in the GAC in our SharePoint server. And we need to modify the Web.config to include our assembly in the following section:

 <System.Workflow.ComponentModel.WorkflowCompiler>
   <authorizedTypes>
       ......
     <authorizedType Assembly="IdeSeg.SharePoint.Workflow.Activities, Version=1.0.0.0, Culture=neutral, PublicKeyToken=3bba710be857fdc1" 
                     Namespace="IdeSeg.SharePoint.Workflow.Activities" 
                     TypeName="*" 
                     Authorized="True" />
   </authorizedTypes>
 </System.Workflow.ComponentModel.WorkflowCompiler>

 Now we  need to modify the file WSS.ACTIONS that we saw  in the first article in order to add our new action

 <Action Name="Log en visor de eventos"
         ClassName="IdeSeg.SharePoint.Workflow.Activities.LogEventViewer"
         Assembly="IdeSeg.SharePoint.Workflow.Activities, Version=1.0.0.0, Culture=neutral, PublicKeyToken=3bba710be857fdc1"
         AppliesTo="all"
         Category="Personalizadas">
    <RuleDesigner Sentence="Logear el evento siguiente %1">
      <FieldBind Field="TextLog" Text="este mensaje" Id="1" DesignerType="TextArea"/>
    </RuleDesigner>
    <Parameters>
      <Parameter Name="TextLog" Type="System.String, mscorlib" Direction="In" />
    </Parameters>
</Action>

What we have done in the first place  is  authorize our assembly  so that  he is now an integral part of the  WorkFlow engine of SharePoint, and secondly, adding it  to the WSS.ACTIONS file, we have indicated to  the SharePoint Designer that  we have added a new activity.

When we use SharePoint Designer to publish a site, at the moment that we created a new workflow or we published  an existing one,  he communicates with SharePoint and recovers the File WSS.ACTIONS  to configurate the assistant of  the WorkFlow.

In this way the new actions will  be part of the file XOML that  the SharePoint Designer will create. 
Finally the result within the SharePoint Designer will be as follow: