Advantage of making a List to a ReadOnlyCollection or AsReadOnly

Question

List<string> dinosaurs = new List<string>();
dinosaurs.Add("Tyrannosaurus");
dinosaurs.Add("Amargasaurus");
dinosaurs.Add("Deinonychus");
dinosaurs.Add("Compsognathus");

1. Why should I use a ReadOnlyCollection as follows:

   var readOnlyDinosaurs = new ReadOnlyCollection<string>(dinosaurs);
   

   instead of:

   dinosaurs.AsReadOnly();
   

2. What is the real advantage of making a List to a ReadOnlyCollection or a AsReadOnly?

Answer 1

In general, they are the same, as mentioned by Erwin. There is one important case where they differ though. Because the AsReadOnly method is generic, the type of the newly created ReadOnlyCollection is infered, not specifically listed. Normally this just saves you a bit of typing, but in the case of anonymous types it actually matters. If you have a list of anonymous objects you need to use AsReadOnly, rather than new ReadOnlyCollection<ATypeThatHasNoName>.

Answer 2

There is no difference, if you look at the code of AsReadOnly():

public ReadOnlyCollection<T> AsReadOnly()
{
    return new ReadOnlyCollection<T>(this);
}