C# - Is it possible to extend an existing built-in class with a new Interface

Question

I am just learning C# and I have a problem now. :-) In C++ I loved to use "const reference" as a parameter to avoid that the called method changes my passed object.

I read somewhere that I can do sth. similar in C# by using Interfaces. In the interface I would just put some "getters" to allow the method a readonly access to my object.

Now guess, that I want to pass my C# method a built-in container like "List". But I want to avoid that the method changes something in that list. Just read only!

My first thought was: - I create a new Interface called IMyOwnInterface, which uses the interface IList as well

• My new interface IMyOwnInterface contains only "getters"

• I change my method to sth. like that MyLittleMethod(IMyOwnInterface if)

• Now the method "MyLittleMethod" can just see the "getters", which I put in my own interface and not the "setters" of IList

Is this possible? Can someone give me a hint?

Answer 1

You want to use List.AsReadOnly.

http://msdn.microsoft.com/en-us/library/e78dcd75(VS.80).aspx

Answer 2

Yes, I believe that would work. Give it a shot, and let us know how it works :D

Answer 3

The answer is no:

IList does not implement IMyOwnInterface so you can't cast it.

IList list = ...
MyLittleMethod((IMyOwnInterface)list)    // Compiler errror here

In this case you can use ReadOnlycollection:

IList list = ...
MyLittleMethod(new ReadOnlyCollection(list))

Answer 4

With interfaces, it's basically only possible through inheritance or with some proxy stuff (either explicit with an adapter class, via a RealProxy implementation, or with a library to do things like that, such as LinFu).

However, out of the box you can use delegates to do late-binding for single methods with matching signature.