private void AddStartandEndRisers(Stairs stairs)
{
    ICollection<ElementId> runIds = stairs.GetStairsRuns();

    foreach (ElementId runId in runIds)
    {
        StairsRun run = stairs.Document.GetElement(runId) as StairsRun;
        if (null != run)
        {
            run.BeginsWithRiser = true;
            run.EndsWithRiser = true;
        }
    }
}

Private Sub AddStartandEndRisers(stairs As Stairs)
    Dim runIds As ICollection(Of ElementId) = stairs.GetStairsRuns()

    For Each runId As ElementId In runIds
        Dim run As StairsRun = TryCast(stairs.Document.GetElement(runId), StairsRun)
        If run IsNot Nothing Then
            run.BeginsWithRiser = True
            run.EndsWithRiser = True
        End If
    Next
End Sub