Perform a taxonomy path based search with the Ektron 8.7 SearchManager class. In this example, I have 3 dropdown lists with optional category filters on a search. The value of the dropdown list item is a taxonomy path. When multiple dropdowns are selected, it is searching with AND based logic.

gistfile1.cs

//create advanced search
AdvancedSearchCriteria criteria = new AdvancedSearchCriteria();
criteria.OrderBy = new List<OrderData>()
    {
        new OrderData(SearchContentProperty.DateCreated, OrderDirection.Descending)
    };
criteria.PagingInfo = new PagingInfo(500);
criteria.PagingInfo.CurrentPage = 1;
criteria.ReturnProperties = new HashSet<PropertyExpression>()
    {
        SearchContentProperty.Id,
        SearchContentProperty.Title,
        SearchContentProperty.QuickLink,
        SearchContentProperty.DateCreated
    };

//build search expression tree with some basics i.e. non-user content, default language
ContentManager contentManager = new ContentManager();
Expression expressionTree = Ektron.Cms.Search.SearchType.IsNonUserContent();
expressionTree &= SearchContentProperty.Language.EqualTo(contentManager.RequestInformation.DefaultContentLanguage);

//filter one
if (!String.IsNullOrEmpty(DdlFilterOne.SelectedValue))
    expressionTree &= SearchContentProperty.TaxonomyCategory.Contains(DdlFilterOne.SelectedValue);

//filter two
if (!String.IsNullOrEmpty(DdlFilterTwo.SelectedValue))
    expressionTree &= SearchContentProperty.TaxonomyCategory.Contains(DdlFilterTwo.SelectedValue);

//filter three
if (!String.IsNullOrEmpty(DdlFilterThree.SelectedValue))
    expressionTree &= SearchContentProperty.TaxonomyCategory.Contains(DdlFilterThree.SelectedValue);

//convert expression tree to criteria
criteria.ExpressionTree = expressionTree;

//perform search
ISearchManager manager = ObjectFactory.GetSearchManager();
SearchResponseData response = manager.Search(criteria);

//do whatever you want with the response.Results here!