public string ID {get;}

string filterID1 = filter1.ID;
var filter = bsl.GetFilter(filterID1);
//or via Linq
//var filter = bsl.GetFilters().FirstOrDefault(f => f.ID == filterID1);

//Must be called on the MCT

//refer to "Query Building Scene Layer for available Types and Values"
//...
//var bsl = ...;
//At 2.x
//var dict = bsl.QueryAvailableFieldsAndValues();

//var dict = bsl.GetAvailableFieldsAndValues();
//var categories = dict.SingleOrDefault(kvp => kvp.Key == "Category").Value;
//var floors = dict.SingleOrDefault(kvp => kvp.Key == "BldgLevel").Value;

//Make a new filter definition
var fd = new FilterDefinition()
{
  Name = "Floor and Category Filter",
  Description = "Example filter",
};
//Set up the values for the filter
var filtervals = new Dictionary<string, List<string>>();
filtervals.Add("BldgLevel", new List<string>() { floors[0] });
var category_vals = categories.Where(v => v == "Walls" || v == "Stairs").ToList() ?? new List<string>();
if (category_vals.Count() > 0)
{
  filtervals.Add("Category", category_vals);
}
//Create a solid block (other option is "Wireframe")
var fdef = new FilterBlockDefinition()
{
  FilterBlockMode = Object3DRenderingMode.None,
  Title = "Solid Filter",
  SelectedValues = filtervals//Floor and Category
};
//Apply the block
fd.FilterBlockDefinitions = new List<FilterBlockDefinition>() { fdef };
//Add the filter definition to the layer
//At 2.x - bsl.SetFilter(fd);
bsl.UpdateFilter(fd);
//Set it active. The ID is auto-generated
bsl.SetActiveFilter(fd.ID);

//var bsl = ...;
//Note: Use HasFilter to check if a given filter id exists in the layer
//Must be called on the MCT
if (bsl.HasFilter(filter1.ID))
  bsl.RemoveFilter(filter1.ID);
//Or remove all filters
bsl.RemoveAllFilters();