IList array and Collection is read-only ERROR

Question

During fixing bug in one project find interesting issue

IList<int> a =new List<int>();
var b = new int[2];
b[0] = 1;
b[1] = 2;
a = b;
a.Clear();

This code is throws exception on a.Clear(); I know how to fix it but I didn't clearly get all steps which leads to this NotSupported exception. And why compiler didn't throws compile time error?

@SrinivasReddyThatiparthy - It's nothing to do with it being an interface type. — Enigmativity, Nov 06 '13 at 10:06
@Enigmativity if `a` was `List` rather than `IList` it would not have worked. — Magnus, Nov 06 '13 at 10:08
@Magnus - It's nothing to do with `IList`. It's because the underlying type is an `int[]`. If the underlying type were `List` it would have worked fine despite being an `IList`. — Enigmativity, Nov 06 '13 at 10:13
@Enigmativity What I meant was that if `a` var declared as `List a = new List();` There would be a compilation error on `a = b;` — Magnus, Nov 06 '13 at 10:15

score 12 · Answer 1 · edited May 23 '17 at 12:09

Yes, this is a somewhat annoying feature of standard C# arrays: They implement IList<>, as defined by the C# language.

Because of this, you can assign a standard C# array to an IList<> type, and the compiler will not complain (because according to the language, an array IS-A IList<>).

Alas, this means that you can then try to do something to change the array such as IList<>.Clear() or IList<>.Add() and you will get a runtime error.

For some discussion about why the language is defined like this, see the following thread:

Why array implements IList?

IList array and Collection is read-only ERROR

1 Answers1