问题
As I mentioned in this post, I faced a for me not understandable compiler behaviour.
The code:
IEnumerable<IList<MyClass>> myData = //...getMyData
foreach (MyClass o in myData){}
It compiles, but fails on runtime: InvalidCastException; for me it is obvious.
If I change the IList to List as following, it complains:
IEnumerable<List<MyClass>> myData = //...getMyData
foreach (MyClass o in myData){}
When instead of the class type I put var as following, intellisense recognizes the correct type:
IEnumerable<List<MyClass>> myData = //...getMyData
foreach (var o in myData){}
My first question was: Why doesn't the compiler complain? The answer was that the behaviour respects the C# Language definition. See chapter 6.2.4 Explicit reference conversions, page 116.
Read the 4th and 5th statement:
• From any interface-type S to any class-type T, provided T is not sealed or provided T implements S.
• From any interface-type S to any interface-type T, provided S is not derived from T.
For the second part of the first statement provided T implements S is clear, no doubts.
But why might we cast an interface-type S to any class-type T if it is not derived or not implemented?
In which case/scenario with an non empty list would the code run without throwing an InvalidCastException?
来源：https://stackoverflow.com/questions/53591187/explanation-for-c-sharp-language-specification-6-2-4-explicit-reference-convers