Question

For declaration perspective the following is allowed

    IList<string> list= new string[3];
    list.Add("Apple");
    list.Add("Manago");
    list.Add("Grapes");

1)
It compiles fine,But runtime i am getting "Collection was of fixed size" error.
Ofcourse ,collection is dynamically grown by size,why did such declaration is accepted by complier ?

2) What are the different lists that i can assign to IList ?
Example

IList<string> fruits=new List<string>();

Here I am assigning List to IList ,What are the various collection
classes can i assign to IList?

Answer 1

The underlying problem here is that System.Array violates the substitution principle by implementing IList<T>. A System.Array type has a fixed size which cannot be changed. The Add method on IList<T> is intended to add a new element to the underlying collection and grow it’s size by 1. This is not possible for a System.Array and hence it throws.

What System.Array really wants to implement here is a read only style IList<T>. Unfortunately no such type exists in the framework and hence it implements the next best thing: IList<T>.

As to the question about what types are assignable to IList<T>, there are actually quite a few including: ReadOnlyCollection<T> and Collection<T>. The list is too long to put here. The best way to see it all is to open IList<T> in reflector and look for derived types of IList<T>.