LoadCombination CreateLoadCombinationLoadCaseLoadUsageLoadNatureAndLoadComponent(Document document)
{
    // Create a new load combination
    LoadCombination loadCombination = LoadCombination.Create(document, "DL1 + RAIN1", LoadCombinationType.Combination, LoadCombinationState.Ultimate);
    if (loadCombination == null)
       throw new Exception("Create new load combination failed.");

    // Get all existing LoadCase
    FilteredElementCollector collector = new FilteredElementCollector(document);
    ICollection<Element> collection = collector.OfClass(typeof(LoadCase)).ToElements();

    // Find LoadCase "DL1"
    LoadCase case1 = null;
    foreach (Element e in collection)
    {
       LoadCase loadCase = e as LoadCase;
       if (loadCase.Name == "DL1")
       {
          case1 = loadCase;
          break;
       }
    }

    // Get all existing LoadNature
    collector = new FilteredElementCollector(document);
    collection = collector.OfClass(typeof(LoadNature)).ToElements();

    // Find LoadNature "Dead"
    LoadNature nature1 = null;
    foreach (Element e in collection)
    {
       LoadNature loadNature = e as LoadNature;
       if (loadNature.Name == "Dead")
       {
          nature1 = loadNature;
          break;
       }
    }

    // Create LoadNature "Dead" if not exist
    if (nature1 == null)
       nature1 = LoadNature.Create(document, "Dead");

    // Create LoadCase "DL1" if not exist
    if (case1 == null)
       case1 = LoadCase.Create(document, "DL1", nature1.Id, LoadCaseCategory.Dead);

    // Create LoadNature "Rain"
    LoadNature nature2 = LoadNature.Create(document, "Rain");
    if (nature2 == null)
       throw new Exception("Create new load nature failed.");

    // Create LoadCase "RAIN1"
    LoadCase case2 = LoadCase.Create(document, "RAIN1", nature2.Id, LoadCaseCategory.Snow);
    if (case1 == null || case2 == null)
       throw new Exception("Create new load case failed.");

    // Create LoadComponents - they consist of LoadCases or nested LoadCombination and Factors
    List<LoadComponent> components = new List<LoadComponent>();
    components.Add(new LoadComponent(case1.Id, 2.0));
    components.Add(new LoadComponent(case2.Id, 1.5));

    // Add components to combination
    loadCombination.SetComponents(components);

    // Create LoadUsages
    LoadUsage usage1 = LoadUsage.Create(document, "Frequent");
    LoadUsage usage2 = LoadUsage.Create(document, "Rare");

    if (usage1 == null || usage2 == null)
       throw new Exception("Create new load usage failed.");

    // Add load usages to combination
    loadCombination.SetUsageIds(new List<ElementId>() {usage1.Id, usage2.Id});

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

    return loadCombination;
}

Private Function CreateLoadCombinationLoadCaseLoadUsageLoadNatureAndLoadComponent(document As Document) As LoadCombination
    ' Create a new load combination
    Dim loadCombination__1 As LoadCombination = LoadCombination.Create(document, "DL1 + RAIN1", LoadCombinationType.Combination, LoadCombinationState.Ultimate)
    If loadCombination__1 Is Nothing Then
        Throw New Exception("Create new load combination failed.")
    End If

    ' Get all existing LoadCase
    Dim collector As New FilteredElementCollector(document)
    Dim collection As ICollection(Of Element) = collector.OfClass(GetType(LoadCase)).ToElements()

    ' Find LoadCase "DL1"
    Dim case1 As LoadCase = Nothing
    For Each e As Element In collection
        Dim loadCase__2 As LoadCase = TryCast(e, LoadCase)
        If loadCase__2.Name = "DL1" Then
            case1 = loadCase__2
            Exit For
        End If
    Next

    ' Get all existing LoadNature
    collector = New FilteredElementCollector(document)
    collection = collector.OfClass(GetType(LoadNature)).ToElements()

    ' Find LoadNature "Dead"
    Dim nature1 As LoadNature = Nothing
    For Each e As Element In collection
        Dim loadNature__3 As LoadNature = TryCast(e, LoadNature)
        If loadNature__3.Name = "Dead" Then
            nature1 = loadNature__3
            Exit For
        End If
    Next

    ' Create LoadNature "Dead" if not exist
    If nature1 Is Nothing Then
        nature1 = LoadNature.Create(document, "Dead")
    End If

    ' Create LoadCase "DL1" if not exist
    If case1 Is Nothing Then
        case1 = LoadCase.Create(document, "DL1", nature1.Id, LoadCaseCategory.Dead)
    End If

    ' Create LoadNature "Rain"
    Dim nature2 As LoadNature = LoadNature.Create(document, "Rain")
    If nature2 Is Nothing Then
        Throw New Exception("Create new load nature failed.")
    End If

    ' Create LoadCase "RAIN1"
    Dim case2 As LoadCase = LoadCase.Create(document, "RAIN1", nature2.Id, LoadCaseCategory.Snow)
    If case1 Is Nothing OrElse case2 Is Nothing Then
        Throw New Exception("Create new load case failed.")
    End If

    ' Create LoadComponents - they consist of LoadCases or nested LoadCombination and Factors
    Dim components As New List(Of LoadComponent)()
    components.Add(New LoadComponent(case1.Id, 2.0))
    components.Add(New LoadComponent(case2.Id, 1.5))

    ' Add components to combination
    loadCombination__1.SetComponents(components)

    ' Create LoadUsages
    Dim usage1 As LoadUsage = LoadUsage.Create(document, "Frequent")
    Dim usage2 As LoadUsage = LoadUsage.Create(document, "Rare")

    If usage1 Is Nothing OrElse usage2 Is Nothing Then
        Throw New Exception("Create new load usage failed.")
    End If

    ' Add load usages to combination
    loadCombination__1.SetUsageIds(New List(Of ElementId)() From {
        usage1.Id,
        usage2.Id
    })

    ' Give the user some information
    TaskDialog.Show("Revit", String.Format("Load Combination ID='{0}' created successfully.", loadCombination__1.Id.IntegerValue))

    Return loadCombination__1
End Function