With this code:

IPredicateExpression select = new PredicateExpression(CompanyFields.CompanyGroup != null);

I get the error:

The call is ambiguous between the following methods or properties: 'SD.LLBLGen.Pro.ORMSupportClasses.EntityField.operator !=(SD.LLBLGen.Pro.ORMSupportClasses.EntityField, System.Collections.IList)' and 'SD.LLBLGen.Pro.ORMSupportClasses.EntityField.operator !=(SD.LLBLGen.Pro.ORMSupportClasses.EntityField, SD.LLBLGen.Pro.ORMSupportClasses.Expression)'

Also, how would I add a DISTINCT clause to the predicate? I want all the CompanyFields.CompanyGroup 's which are not null and DISTINCT

Solved it.

Created a Typed List based on the Company then set the allowDuplicates to false on the fill.

CompanyGroupTypedList ds = new CompanyGroupTypedList();
ISortExpression sort = new SortExpression(CompanyFields.CompanyGroup | SortOperator.Ascending);
ds.Fill(0, sort, false);

You should compare with DBNull.Value instead. Comparing with null indeed is ambigious, as the CLR then doesn't know which method to call.