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Tridion Core service - working with a Hierarchical Taxonomy
CoreService2010Client client = new CoreService2010Client();
XElement xmlCategoryKeywords = client.GetListXml(category.Id,
                                                 new KeywordsFilterData());

var categoryKeywords = xmlCategoryKeywords.Elements().Select(element =>
    element.Attribute("ID").Value).Select(id => (KeywordData)client.Read(id, null)
);
foreach (KeywordData keyword in categoryKeywords)
{
    Console.WriteLine("t Keyword ID={0}, Title={1}", keyword.Id, keyword.Title);
}

CoreService2010Client client = new CoreService2010Client("basicHttp_2010");

KeywordsFilterData keywordsDataFilter = new KeywordsFilterData()
{
    BaseColumns = ListBaseColumns.IdAndTitle,
    IsRoot = true
};

UsingItemsFilterData usingItemsFilter = new UsingItemsFilterData()
{
    BaseColumns = ListBaseColumns.IdAndTitle,
    ItemTypes = new[] { ItemType.Keyword },
    InRepository = new LinkToRepositoryData() { IdRef = "tcm:0-1-1" }
};

XElement parents = client.GetListXml("tcm:1-272-512", keywordsDataFilter);

foreach (XElement parent in parents.Descendants())
{
    // Do something with the parent (top level) KW

    XElement children = client.GetListXml(parent.Attribute("ID").Value, usingItemsFilter);

    foreach (XElement child in children.Descendants())
    {
        // Do something with the child KW
    }
}