Fixed Size to List

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

When you call list.Add, you are trying to insert an item to the end of you array. Arrays are a fixed size collection so you can't do an Add. Instead you will have to assign the entries via the indexer:

list[0] = "a";
list[1] = "b";
list[2] = "c";

Answer 2

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>.

Answer 3

@Jake: Actually a string[] IS castable to an IList... but you can't ADD to it.

If you want to pre-populate the list, then you could use something like:

IList<string> list= new string[3] { "Apple", "Mango", "Grapes" };

but then what's the point of making it an IList? You still couldn't add to it. If it really is a fixed-sized list, then make it a string[]. Otherwise, make it a List(), as Jake suggests.