possibly you get a compile error because of FirstOrDefault extension method. It has two forms:
either:
public static TSource FirstOrDefault<TSource>(
this IEnumerable<TSource> source
)
or
public static TSource FirstOrDefault<TSource>(
this IEnumerable<TSource> source,
Func<TSource, bool> predicate
)
First version, simply selects a value from a given IEnumerable. The second one needs a selector function. Your usage doesn't match any of these two. Therefore, a compile error.
Since you don't know how to use it I advice you to check out some samples on msdn and other sites.
documentation is here:
FirstOrDefault[
^]
another advice is to use typed datasets and tables. use the designer of the IDE to create your data structures.
see here:
create dataset[
^]
and a good start point for linq:
LINQ[
^]
If you follow instructions on these pages, you can have deeper understanding of those concepts and tools.
have a good day.