Is there some method in ICollection in C# that would add all elements of another collection? Right now I have to always write foreach cycle for this:

ICollection<Letter> allLetters = ... //some initalization
ICollection<Letter> justWrittenLetters = ... //some initalization
... //some code, adding to elements to those ICollections

foreach(Letter newLetter in justWrittenLetters){
    allLetters.add(newLetter);
}

My question is, is there method that can replace that cycle? Like for example the method addAll(Collection c) in Java ? So I would write only something like:

allLetters.addAll(justWrittenLetters);