c# - ForEach giving Invalid Cast at Runtime -

i have following:

 foreach (itemoption itemoption in p.items.select(e => e.itemoption).groupby(e => e.id))   { // work on itemoptions } 

this compiles. @ runtime invalid cast exception:

unable cast object of type 'grouping[system.string,mynamespace.itemoption]' type 'mynamespace.itemoption'.

if change code to, e.g. string type of item :

 foreach (string itemoption in p.items.select(e => e.itemoption).groupby(e => e.id))   { // work on itemoptions } 

then compiler tells me types incompatible.

why doesn't compiler flag type incompatibility in first code block?

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i did further investigation , found that, given following code:

var foo = p.items.select(e => e.itemoption).groupby(e => e.id)); type singleelementtype = foo.elementat(0).gettype(); 

singleelementtype is:

system.linq.lookup`2+grouping[system.string, mynamespace.itemoption]

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update following on answers have put simpler case demonstrate issue

given objects:

interface imyobj {     string id; }  class myobj : imyobj {     public string id;     public myobj2 cg; }  class myobj2 { } 

this fail @ compile time

ienumerable<myobj> compiletimefaillist = new list<myobj>() foreach (myobj2 myobj2 in compiletimefaillist.where(x => x.id != null)) { 

and fail @ run time

ienumerable<imyobj> runtimefaillist = new list<imyobj>(); foreach (myobj2 myobj2 in runtimefaillist.where(x => x.id != null)) {  

the reason being objects in runtimefaillist may extend myobj2 , cannot determined compiler.

itemoption presumably not sealed class (unlike system.string) it's possible result of p.items.select(...).groupby(...) implementations of igrouping<...> also itemoption values. compiler can't know, inserts implicit cast. string doesn't implement igrouping<...> , sealed, compiler can spot that's mistake.

foreach has always included cast necessary. it's bit nasty, because it's hidden... without it, foreach have been painful use pre-generics.

now why it's actually wrong... each item in result going grouping rather individual item. let me know if need more working results.