#region Autodesk.Revit.DB.Structure.LoadCombination.GetUsageIds()
void ModifyLoadCombinationLoadCaseLoadUsageLoadNatureAndLoadComponent(Document document, LoadCombination loadCombination)
{
    // Change name of LoadCombination
    loadCombination.Name = "DL2 + RAIN1";

    // Get any LoadCase from combination
    // Combination can have assigned LoadCase or other (nested) LoadCombination so we need to filter out any LoadCombination
    LoadCase case1 = null;
    IList<ElementId> caseAndCombinationIds = loadCombination.GetCaseAndCombinationIds();
    foreach (ElementId id in caseAndCombinationIds)
    {
        Element element = document.GetElement(id);
        if (element is LoadCase)
        {
           case1 = (LoadCase)element;
           break;
        }
        else if (element is LoadCombination)
        {
           continue;
        }
    }

    if (case1 == null)
       throw new Exception("Can't get LoadCase.");

    // Change case name and number
    case1.Name = "DL2";
    if (LoadCase.IsNumberUnique(document, 3))
    {
        case1.Number = 3;
    }

    // Create load nature
    LoadNature liveNature = LoadNature.Create(document, "Dead nature");
    if (liveNature == null)
       throw new Exception("Create new load nature failed.");

    // Change nature category, ID and number for case
    case1.SubcategoryId = new ElementId(BuiltInCategory.OST_LoadCasesDead);
    case1.NatureId = liveNature.Id;

    //Change factor for case1
    IList<LoadComponent> components = loadCombination.GetComponents();
    foreach (LoadComponent loadComponent in components)
    {
        if (loadComponent.LoadCaseOrCombinationId == case1.Id)
        {
            loadComponent.Factor = 3.0;
        }
    }

    loadCombination.SetComponents(components);

    // Remove one usage from combination
    IList<ElementId> usages = loadCombination.GetUsageIds();
    usages.RemoveAt(0);
    loadCombination.SetUsageIds(usages);

    // Give the user some information
    TaskDialog.Show("Revit", string.Format("Load Combination ID='{0}' modified successfully.", loadCombination.Id.ToString()));
}
#endregion

#Region "Autodesk.Revit.DB.Structure.LoadCombination.GetUsageIds()"
        Private Sub ModifyLoadCombinationLoadCaseLoadUsageLoadNatureAndLoadComponent(document As Document, loadCombination As LoadCombination)
            ' Change name of LoadCombination
            loadCombination.Name = "DL2 + RAIN1"

            ' Get any LoadCase from combination
            ' Combination can have assigned LoadCase or other (nested) LoadCombination so we need to filter out any LoadCombination
            Dim case1 As LoadCase = Nothing
            Dim caseAndCombinationIds As IList(Of ElementId) = loadCombination.GetCaseAndCombinationIds()
            For Each id As ElementId In caseAndCombinationIds
                Dim element As Element = document.GetElement(id)
                If TypeOf element Is LoadCase Then
                    case1 = DirectCast(element, LoadCase)
                    Exit For
                ElseIf TypeOf element Is LoadCombination Then
                    Continue For
                End If
            Next

            If case1 Is Nothing Then
                Throw New Exception("Can't get LoadCase.")
            End If

            ' Change case name and number
            case1.Name = "DL2"
            If LoadCase.IsNumberUnique(document, 3) Then
                case1.Number = 3
            End If

            ' Create load nature
            Dim liveNature As LoadNature = LoadNature.Create(document, "Dead nature")
            If liveNature Is Nothing Then
                Throw New Exception("Create new load nature failed.")
            End If

            ' Change nature category, ID and number for case
            case1.SubcategoryId = New ElementId(BuiltInCategory.OST_LoadCasesDead)
            case1.NatureId = liveNature.Id

            'Change factor for case1
            Dim components As IList(Of LoadComponent) = loadCombination.GetComponents()
            For Each loadComponent As LoadComponent In components
                If loadComponent.LoadCaseOrCombinationId = case1.Id Then
                    loadComponent.Factor = 3.0
                End If
            Next

            loadCombination.SetComponents(components)

            ' Remove one usage from combination
            Dim usages As IList(Of ElementId) = loadCombination.GetUsageIds()
            usages.RemoveAt(0)
            loadCombination.SetUsageIds(usages)

            ' Give the user some information
            TaskDialog.Show("Revit", String.Format("Load Combination ID='{0}' modified successfully.", loadCombination.Id.ToString()))
        End Sub
#End Region