How about this expression
DtBuild.AsEnumerable.Where(Function(x) Not(String.IsNullOrWhiteSpace(x("column A").ToString)) AndAlso Not(String.IsNullOrWhiteSpace(x("column B").ToString)) AndAlso (CDate(x("Column A").ToString)<=(CDate(x("Column B").ToString)))).ToArray.copytodatatable
Regards
Gokul