とった!
リソースの値は Application.Current.Resources[key] で設定できます。
したがって、私の例では、誰かがこの問題に遭遇した場合、私は何かをします。
MyObservableCollection coll1 = Application.FindResource("resourceName") as MyObservableCollection
foreach(Item i in coll1)
{
if(somecondition){i.someProperty == someValue;}
}
//coll2 does NOT reflect the above change!!!
MyObservableCollection coll2 = Application.FindResource("resourceName") as MyObservableCollection;
Application.Current.Resources["resourceName"] = coll1;
//coll3 DOES reflect the above change
MyObservableCollection coll3 = Application.FindResource("resourceName") as MyObservableCollection