Rewrite this foreach yield to a linq yield?

Question

Say I have the following code (context narrowed down as to keep the question scope limited)

public static IEnumerable<Color> GetThemColors(){
 var ids = GetThePrimaryIds();
 foreach (int id in ids){
  yield return GetColorById(id);
 }
 ids = GetTheOtherIds();
 foreach (int id in ids){
  yield return GetOtherColorsById(id);
 }
}

I would like to rewrite them to something like this (which off course doesn't compile

public static IEnumerable<Color> GetThemColors(){
 GetThePrimaryIds().Select(id=>yield return GetColorById(id));
 GetTheOtherIds().Select(id=>yield return GetOtherColorsById(id));  
}

The key point being that in my first snippet I have two foreach enumerators yielding, which I don't know how to do in linq without loosing my lazy-loading features.

Answer 1

You want Concat:

return GetThePrimaryIds().Select(id => GetColorById(id)).Concat(
    GetTheOtherIds().Select(id => GetOtherColorsById(id)));

Also note that you don't need yield return in lambdas.