myList.ForEach(a => { if (a.Trim().Length == 0) a = "0.0"; })

for (Int32 index = 0; index < myList.Count; index++)
    if (myList[index].Trim().Length == 0)
        myList[index] = "0.0";

myList = (from a in myList
          select (a.Trim().Length == 0) ? "0.0" : a).ToList();

myList = myList.Select(a => (a.Trim().Length == 0) ? "0.0" : a).ToList();

List<Double> myDoubleList =
    (from a in myList
     select (a.Trim().Length == 0 ? "0" : a) into d
     select Double.Parse(d)).ToList();

List<Double> myDoubleList =
    (from a in myList
     select (a.Trim().Length == 0 ? "0.0" : a) into d
     select Double.Parse(d, CultureInfo.InvariantCulture)).ToList();

public static void MutateEach(this IList<T> list, Func<T, T> mutator)
{
    int count = list.Count;
    for (int n = 0; n < count; n++)
        list[n] = mutator(list[n]);
}

myList.MutateEach(a => (a.Trim().Length == 0) ? "0.0" : a);