AssemblyInstance

// Create assembly instance, name the assembly, and create views of the assembly
AssemblyInstance CreateAssemblyAndViews(Autodesk.Revit.DB.Document doc, ICollection<ElementId> elementIds)
{
    AssemblyInstance assemblyInstance = null;
    using (Transaction transaction = new Transaction(doc))
    {
        ElementId categoryId = doc.GetElement(elementIds.First()).Category.Id; // use category of one of the assembly elements
        if (AssemblyInstance.IsValidNamingCategory(doc, categoryId, elementIds))
        {
            transaction.Start("Create Assembly Instance");
            assemblyInstance = AssemblyInstance.Create(doc, elementIds, categoryId);
            transaction.Commit(); // commit the transaction that creates the assembly instance before modifying the instance's name

            if (transaction.GetStatus() == TransactionStatus.Committed)
            {
                transaction.Start("Set Assembly Name");
                assemblyInstance.AssemblyTypeName = "My Assembly Name";
                transaction.Commit();
            }

            if (assemblyInstance.AllowsAssemblyViewCreation()) // create assembly views for this assembly instance
            {
                if (transaction.GetStatus() == TransactionStatus.Committed)
                {
                    transaction.Start("View Creation");
                    View3D view3d = AssemblyViewUtils.Create3DOrthographic(doc, assemblyInstance.Id);
                    ViewSchedule partList = AssemblyViewUtils.CreatePartList(doc, assemblyInstance.Id);
                    transaction.Commit();
                }
            }
        }
    }

    return assemblyInstance;
}