The PgSqlDependency class is designed to watch for changes in tables on the server. This is accomplished using LISTEN ... NOTIFY server side mechanism and recurring check requests.

To start watching for a table in a schema perform the following steps:

1. Create a stored function on the server. This function should issue NOTIFY "table_changed_schemaname.tablename" command, where schemaname and tablename represent names of the schema and the table you want to watch.
2. Create a trigger that executes the stored function every time the table is altered.
3. Create a PgSqlCommand object with CommandText property set to SQL statement like SELECT * FROM schemaname.tablename.
4. Create a PgSqlDependency object and connect it to the PgSqlCommand object (for example, with the AddCommandDependency method).
5. Assign a handler to the OnChange event.
6. Prepare a PgSqlConnection object (or a connection string) to use by the PgSqlDependency object and call the Start method.

Once you perform these steps the the PgSqlDependency object will poll the server periodically, and if the table is modified since the last call, it will invoke your event handler.

Note that you can use single PgSqlDependency instance to watch for several tables. To do this, specify all necessary tables in the FROM clause of command's SQL statement.

Note also that PgSqlDependency can watch for whole tables only. If you restrict the SELECT statement with WHERE clause, it will not affect notifications.

In PostgreSQL 9.0 and higher, you can also get the infiormation about the table change, that is passed as a second parameter of the NOTIFY command.

This class is not available in Mobile Edition.

CREATE OR REPLACE FUNCTION test.note_dept()
  RETURNS TRIGGER AS 
  BEGIN
  PERFORM pg_notify('table_changed_test.dept', TG_OP);
  RETURN NULL;
  END;
 LANGUAGE plpgsql;


CREATE TRIGGER notify_dept AFTER INSERT OR UPDATE OR DELETE
  ON test.dept
  EXECUTE PROCEDURE test.note_dept();

PgSqlConnection connection = new PgSqlConnection("User Id = postgres; Password = postgres; host = localhost; Port = 5432; database = testdb;");
PgSqlDependency depend = new PgSqlDependency();
depend.OnChange += new OnChangeEventHandler(dependency_OnChange);
PgSqlCommand selectCommand = new PgSqlCommand("SELECT * FROM test.dept", connection);
depend.AddCommandDependency(selectCommand);
depend.CheckTimeout = 500;
connection.Open();
try {
  depend.Start(connection);

  //different actions with data and check correct depend work
  PgSqlCommand commandInsert = new PgSqlCommand("INSERT INTO test.dept VALUES (111, 'test', 'test');", connection);
  PgSqlCommand commandUpdate = new PgSqlCommand("UPDATE test.dept SET dname = 'test' WHERE deptno = 10;", connection);
  PgSqlCommand commandDelete = new PgSqlCommand("DELETE FROM test.dept WHERE dname = 'test';", connection);
  commandInsert.ExecuteNonQuery();
  System.Threading.Thread.Sleep(600);
  commandUpdate.ExecuteNonQuery();
  System.Threading.Thread.Sleep(600);
  commandDelete.ExecuteNonQuery();
  System.Threading.Thread.Sleep(600);
}
finally {
  depend.Stop();
  connection.Close();
   }

...

private void dependency_OnChange(object sender, PgSqlTableChangeEventArgs e) {

  MessageBox.Show("Table changed: " + e.TableName + "; Operation: " + e.Info);
}

Dim connection As New PgSqlConnection("User Id = postgres; Password = postgres; host = localhost; Port = 5432; database = postgres;")
Dim depend As New PgSqlDependency
AddHandler depend.OnChange, New OnChangeEventHandler(AddressOf Me.dependency_OnChange)
        Dim selectCommand As New PgSqlCommand("SELECT * FROM test.dept", connection)
        depend.AddCommandDependency(selectCommand)
        depend.CheckTimeout = 500
        connection.Open()
        Try
                depend.Start(connection)
                Dim commandInsert As New PgSqlCommand("INSERT INTO test.dept VALUES (111, 'test', 'test');", connection)
                Dim commandUpdate As New PgSqlCommand("UPDATE test.dept SET dname = 'test' WHERE deptno = 10;", connection)
                Dim commandDelete As New PgSqlCommand("DELETE FROM test.dept WHERE dname = 'test';", connection)
                commandInsert.ExecuteNonQuery()
                System.Threading.Thread.Sleep(600)
                commandUpdate.ExecuteNonQuery()
                System.Threading.Thread.Sleep(600)
                commandDelete.ExecuteNonQuery()
                System.Threading.Thread.Sleep(600)
        Finally
                depend.Stop()
                connection.Close()
        End Try

        ...

        Private Sub dependency_OnChange(ByVal sender As Object, ByVal e As PgSqlTableChangeEventArgs)

        MessageBox.Show("Table changed " & e.TableName & "; Operation: " & e.Info)
End Sub

System.Object
   Devart.Data.PostgreSql.PgSqlDependency

Reference

PgSqlDependency Members
Devart.Data.PostgreSql Namespace