MaxBy(this IEnumerable<T>) and MinBy(this IEnumerable<T>) don't return IEnumerable<T> - they return single elements (i.e. T), so just do this:

double topPriceInList = prices.MaxBy(h => h.High).High;

That said, that code above doesn't need MaxBy, just use Max:

double topPriceInList = prices.Max(h => h.High);

// or:

double topPriceInList = prices.Select(h => h.High).Max();

• Note that Min, Max, MinBy, and MaxBy will all throw an exception at runtime if the source collection is empty (or any previous .Where steps eliminated all elements)
• ...so you may need to use MinOrDefault, MaxOrDefault, MaxByOrDefault, or MaxByOrDefault, in which case use the null-safe navigation operator ?. (aka elvis-operator).
  • But make sure the ...OrDefault() extension-method is invoked on IEnumerable<Nullable<T>> and not IEnumerable<T> because default(T) for value-types is never null which is a frequent source of bugs.

double? topPriceInListOrNull = prices.Select(h => (double?)h.High).MaxOrDefault();