Collection<T> does not Contain the all properties of ICollection interface

Question

The Collection<T> class implements multiple interfaces, one of them being ICollection. The ICollection interface has 2 properties that are not implemented in Collection<T>.

In C# I believe you have to implement all the methods and properties of an Interface in the class that inherits it. So how is Collection<T> class allowed to get away with it?

Answer 1

This is called explicitly implementing an interface. You can make a member not visible from the outside unless the object reference is converted to the interface type.

In the context of Collection<T> implementing ICollection, the interface in question defines legacy methods that existed before generics were introduced to C#. You could say it is the 'old ugly' way of managing collections.

The implementors decided to hide these ugly methods, while still providing its functionality to the caller.

var x = new Collection<int>();
object syncRoot = x.SyncRoot; //CS1061: Collection<int> does not contain a ....
ICollection collection = x;
syncRoot = collection.SyncRoot; //ok

Another scenario is where there is a conflict, usually due to external interfaces that are not well designed, and cannot be changed. An example:

interface IFile
{
    void Save();
}
interface IDatabaseRecord
{
    void Save();
}

class Customer : IFile, IDatabaseRecord
{
    public void Save()
    {
        //what to do here?
    }
}

This may be overcome by implementing the method explicitly:

class Customer : IFile, IDatabaseRecord
{
    void IFile.Save() { }
    void IDatabaseRecord.Save() { }
}

Note that this is almost always a code smell - it could confuse the caller into thinking a method is not present, or calling the wrong implementation (which caused this question to be posted in the first place).