Arrays and the generic IList interface

A one-dimensional array T[] implements the interface System.Collections.Generic.IList<T> (IList<T> for short) and its base interfaces. Accordingly, there is an implicit conversion from T[] to IList<T> and its base interfaces. In addition, if there is an implicit reference conversion from S to T then S[] implements IList<T> and there is an implicit reference conversion from S[] to IList<T> and its base interfaces (§6.1.6). If there is an explicit reference conversion from S to T then there is an explicit reference conversion from S[] to IList<T> and its base interfaces (§6.2.4). For example:

using System.Collections.Generic;

class Test
{
static void Main() {
string[] sa = new string[5];
object[] oa1 = new object[5];
object[] oa2 = sa;

IList<string> lst1 = sa; // Ok
IList<string> lst2 = oa1; // Error, cast needed
IList<object> lst3 = sa; // Ok
IList<object> lst4 = oa1; // Ok

IList<string> lst5 = (IList<string>)oa1; // Exception
IList<string> lst6 = (IList<string>)oa2; // Ok
}
}

The assignment lst2 = oa1 generates a compile-time error since the conversion from object[] to IList<string> is an explicit conversion, not implicit. The cast (IList<string>)oa1 will cause an exception to be thrown at run-time since oa1 references an object[] and not a string[]. However the cast (IList<string>)oa2 will not cause an exception to be thrown since oa2 references a string[].

Whenever there is an implicit or explicit reference conversion from S[] to IList<T>, there is also an explicit reference conversion from IList<T> and its base interfaces to S[] (§6.2.4).

When an array type S[] implements IList<T>, some of the members of the implemented interface may throw exceptions. The precise behavior of the implementation of the interface is beyond the scope of this specification.