Why SomethingCollection is used instead of Collection<Something>?

Question

Hi,

In .NET Framework, there are some classes which use SomethingCollection syntax.

For example, when dealing with SqlCommand, it has a parameter Parameters of type SqlParameterCollection. Since it does not have a form of IEnumerable<SqlParameter> (or IList<SqlParameter> or something similar), it is impossible to write:

foreach (var c in sqlCommand.Parameters)
{
    Debug.WriteLine(string.Concat(c.ParameterName, ": ", c.Value));
}

Instead, var must be replaced by an explicit SqlParameter.

The thing is very similar when dealing with Windows Forms controls (collections of listview items, etc.), and exists in other classes.

What is this thing? Why such classes exist and are used instead of generic IEnumerables/ILists?

Answer 1

Many of the collections exposed by .NET APIs were designed before .NET 2.0, thus, before generics were available.

Answer 2

It is possible that certain parts of the .net framework were designed by different teams with different goals in mind.

Some practitioners of Domain Driven Design would advocate the use of aggregations, which are root objects that control all the objects in the collections, as a form of a stop gap for some issues that arise from exposing collections.

I am guessing the parts of the .net framework that use this design had a reason for restricting access.