How do I write a filter which does WHERE field1=3 and (field2 = 1 or field2 is null)?

IRelationPredicateBucket filter = new RelationPredicateBucket();
filter.PredicateExpression.Add(SomeEntityFields.Field1 == 3);

PredicateExpression subFilter = new PredicateExpression();
subFilter.Add(SomeEntityFields.Field2 == 1);
subFilter.AddWithOr(SomeEntityFields.Field2 == System.DBNull.Value);
filter.PredicateExpression.Add(subFilter);

or

IRelationPredicateBucket filter = new RelationPredicateBucket();
filter.PredicateExpression.Add(SomeEntityFields.Field1 == 3);
filter.PredicateExpression.Add((SomeEntityFields.Field2 == 1) | (SomeEntityFields.Field2 == System.DBNull.Value));

or

IRelationPredicateBucket filter = new RelationPredicateBucket();
filter.PredicateExpression.Add( (SomeEntityFields.Field1 == 3) & 
     ((SomeEntityFields.Field2 == 1) | (SomeEntityFields.Field2 == System.DBNull.Value)) );

For more info, read The Predicate System.

Thank you so much! It's so kind of you!