public class Application_DocumentOpened : IExternalApplication
{
    /// <ExampleMethod>
    /// <summary>
    /// Implement this method to subscribe to event.
    /// </summary>
    public Result OnStartup(UIControlledApplication application)
    {
        try
        {
            // Register event. 
           application.ControlledApplication.DocumentOpened += new EventHandler
                <Autodesk.Revit.DB.Events.DocumentOpenedEventArgs>(application_DocumentOpened);
        }
        catch (Exception)
        {
            return Result.Failed;
        }

        return Result.Succeeded;
    }

    /// <summary>
    /// Implement OnShutdown method of IExternalApplication interface to unregister subscribed event
    /// </summary>
    public Result OnShutdown(UIControlledApplication application)
    {
        // remove the event.
       application.ControlledApplication.DocumentOpened -= application_DocumentOpened;
        return Result.Succeeded;
    }

    /// <TrivialCode>
    /// Code ID: 501
    /// For DocumentOpened class description
    /// </TrivialCode>

    /// <description>
    /// This sample demonstrates how to subscribe to the DocumentOpened event and modify the model in the event handler method. 
    /// </description>
    /// <summary>
    /// This is the event handler. We modify the model.
    /// </summary>
    /// <param name="sender">Event sender.</param>
    /// <param name="args"></param>
    public void application_DocumentOpened(object sender, DocumentOpenedEventArgs args)
    {
        // get document from event args.
        Document doc = args.Document;

        // Following code snippet demonstrates support of DocumentOpened event to modify the model.
        // Because DocumentOpened supports model changes, it allows user to update document data.
        // Here, this sample assigns a specified value to ProjectInformation.Address property. 
        // User can change other properties of document or create(delete) something as he likes.
        // 
        // Please note that ProjectInformation property is empty for family document.
        // So please don't run this sample on family document.
        using (Transaction transaction = new Transaction(doc, "Edit Address"))
        {
           if (transaction.Start() == TransactionStatus.Started)
           {
              doc.ProjectInformation.Address =
                  "United States - Massachusetts - Waltham - 1560 Trapelo Road";
              transaction.Commit();
           }
        }
    }

    /// </ExampleMethod>
}

Public Class Application_DocumentOpened
    Implements IExternalApplication
    ' <ExampleMethod>
    ' <summary>
    ' Implement this method to subscribe to event.
    ' </summary>
    Public Function OnStartup(application As UIControlledApplication) As Autodesk.Revit.UI.Result Implements IExternalApplication.OnStartup
        Try
            ' Register event. 
            AddHandler application.ControlledApplication.DocumentOpened, AddressOf application_DocumentOpened
        Catch generatedExceptionName As Exception
            Return Result.Failed
        End Try

        Return Result.Succeeded
    End Function

    ' <summary>
    ' Implement OnShutdown method of IExternalApplication interface to unregister subscribed event
    ' </summary>
    Public Function OnShutdown(application As UIControlledApplication) As Autodesk.Revit.UI.Result Implements IExternalApplication.OnShutdown
        ' remove the event.
        RemoveHandler application.ControlledApplication.DocumentOpened, AddressOf application_DocumentOpened
        Return Result.Succeeded
    End Function

    ' <TrivialCode>
    ' Code ID: 501
    ' For DocumentOpened class description
    ' </TrivialCode>


    ' <description>
    ' This sample demonstrates how to subscribe to the DocumentOpened event and modify the model in the event handler method. 
    ' </description>
    ' <summary>
    ' This is the event handler. We modify the model.
    ' </summary>
    ' <param name="sender">Event sender.</param>
    ' <param name="args"></param>
    Public Sub application_DocumentOpened(sender As Object, args As DocumentOpenedEventArgs)
        ' get document from event args.
        Dim doc As Document = args.Document

        ' Following code snippet demonstrates support of DocumentOpened event to modify the model.
        ' Because DocumentOpened supports model changes, it allows user to update document data.
        ' Here, this sample assigns a specified value to ProjectInformation.Address property. 
        ' User can change other properties of document or create(delete) something as he likes.
        '
        ' Please note that ProjectInformation property is empty for family document.
        ' So please don't run this sample on family document.
        Using transaction As New Transaction(doc, "Edit Address")
            If transaction.Start() = TransactionStatus.Started Then
                doc.ProjectInformation.Address = "United States - Massachusetts - Waltham - 1560 Trapelo Road"
                transaction.Commit()
            End If
        End Using
    End Sub

    ' </ExampleMethod>
End Class