public void SetActiveFilter( 
   string filterID
)

{
  // Note: call within QueuedTask.Run()
  {
    //Note: Use HasFilter to check if a given filter id exists in the layer
    if (bsl.HasFilter(filter.ID))
      bsl.SetActiveFilter(filter.ID);
    var activeFilter = bsl.GetActiveFilter();

    //Clear the active filter
    bsl.ClearActiveFilter();
  }
}

{
  // Note: call within QueuedTask.Run()
  {
    //refer to "Query Building Scene Layer for available Types and Values
    var dict = bsl.GetAvailableFieldsAndValues();
    var categories = dict.SingleOrDefault(kvp => kvp.Key == "Category").Value;
    //get a list of existing floors or "levels"
    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
    bsl.UpdateFilter(fd);
    //Set it active. The ID is auto-generated
    bsl.SetActiveFilter(fd.ID);
  }
}