This code compiles and gives runtime errors, even if it would be easy for a compiler to figure this out:

interface II{}
class A{}

IEnumerable<II> enumerable=...;

foreach(A a in enumerable)
....

class A is no base class of II, it does not even implement it. So the compiler uses the c# 1.0-enumerable-interface which returns an object, and then tries to cast that object into an A, which - suprise - does fail and causes an runtime exception.

Maybe such behaviour is useful in edge cases, but in normal scenarios it just triggers runtime errors. During refactoring most of the time the compiler will help, what you can do, and what not, but not in this case.

Imagine you usually use class A, and at some places an IEnumerable<A>. Now you try to refacture your code, and replace most uses of A by an Interface IA, which A implements. IEnumerable<A> is replaced by IEnumerable<IA> and everything seems to work, until sometime you introduce a new class LeanA:IA.

then the line
IEnumerable<IA> ias=...
foreach(A a in ias)

will break with an runtime exception.
It would be nice, to have at least a compiler switch to disable this strange behabviour.