Lucene.Net.Search.BooleanFilter.Add C# (CSharp) Method

        public void Add(FilterClause filterClause)
        {
            if (filterClause.Occur == Occur.MUST)
            {
                if (mustFilters == null)
                {
                    mustFilters = new EquatableList<Filter>();
                }
                mustFilters.Add(filterClause.Filter);
            }
            if (filterClause.Occur == Occur.SHOULD)
            {
                if (shouldFilters == null)
                {
                    shouldFilters = new EquatableList<Filter>();
                }
                shouldFilters.Add(filterClause.Filter);
            }
            if (filterClause.Occur == Occur.MUST_NOT)
            {
                if (notFilters == null)
                {
                    notFilters = new EquatableList<Filter>();
                }
                notFilters.Add(filterClause.Filter);
            }
        }

 public void TestCombinedNullDocIdSets()
 {
     BooleanFilter booleanFilter = new BooleanFilter();
     booleanFilter.Add(GetTermsFilter(@"price", @"030"), BooleanClause.Occur.MUST);
     booleanFilter.Add(GetNullDISFilter(), BooleanClause.Occur.MUST);
     TstFilterCard(@"A MUST filter that returns a null DIS should never return documents", 0, booleanFilter);
     booleanFilter = new BooleanFilter();
     booleanFilter.Add(GetTermsFilter(@"price", @"030"), BooleanClause.Occur.MUST);
     booleanFilter.Add(GetNullDISIFilter(), BooleanClause.Occur.MUST);
     TstFilterCard(@"A MUST filter that returns a null DISI should never return documents", 0, booleanFilter);
     booleanFilter = new BooleanFilter();
     booleanFilter.Add(GetTermsFilter(@"price", @"030"), BooleanClause.Occur.SHOULD);
     booleanFilter.Add(GetNullDISFilter(), BooleanClause.Occur.SHOULD);
     TstFilterCard(@"A SHOULD filter that returns a null DIS should be invisible", 1, booleanFilter);
     booleanFilter = new BooleanFilter();
     booleanFilter.Add(GetTermsFilter(@"price", @"030"), BooleanClause.Occur.SHOULD);
     booleanFilter.Add(GetNullDISIFilter(), BooleanClause.Occur.SHOULD);
     TstFilterCard(@"A SHOULD filter that returns a null DISI should be invisible", 1, booleanFilter);
     booleanFilter = new BooleanFilter();
     booleanFilter.Add(GetTermsFilter(@"price", @"030"), BooleanClause.Occur.MUST);
     booleanFilter.Add(GetNullDISFilter(), BooleanClause.Occur.MUST_NOT);
     TstFilterCard(@"A MUST_NOT filter that returns a null DIS should be invisible", 1, booleanFilter);
     booleanFilter = new BooleanFilter();
     booleanFilter.Add(GetTermsFilter(@"price", @"030"), BooleanClause.Occur.MUST);
     booleanFilter.Add(GetNullDISIFilter(), BooleanClause.Occur.MUST_NOT);
     TstFilterCard(@"A MUST_NOT filter that returns a null DISI should be invisible", 1, booleanFilter);
 }