Casting object to IEnumerable<object> [duplicate]

Question

The following is a simplified version of the problem I'm having:

var list = new List<int> {1, 2, 3, 4, 5};
// list Count = 5 System.Collections.Generic.List<int>

var obj = list as object;
// obj  Count = 5   object {System.Collections.Generic.List<int>}

var enumerable = obj as IEnumerable<object>;
// enumerable   null    System.Collections.Generic.IEnumerable<object>

The comments are the values from the watch window in Visual Studio 2017

However if I enter:

obj as IEnumerable<object>

in the watch window, I get:

obj as IEnumerable<object>  Count = 5   System.Collections.Generic.IEnumerable<object> {System.Collections.Generic.List<int>}

So how come I can cast it in the watch window and it works, but if I do it in the code it evaluates to null?

Answer 1

From the C# documentation:

Variance in generic interfaces is supported for reference types only. Value types do not support variance. For example, IEnumerable<int> cannot be implicitly converted to IEnumerable<object>, because integers are represented by a value type.

What helps to know here is that the following two things are very different operations under the hood:

var l = (object) new List<int>();
var i = (object) 42;

despite the syntax looking the same. The first line merely treats the list as another type (with some checks whether the cast actually can work), while the second converts the integer into an object containing it (boxing). There's a third variant, which calls a conversion operator, and still looks the same. Since the first variant can get away with changing nothing about the object and just interpreting it differently it's vastly cheaper than the second. And I'd guess in this context here the only option that can work for interface covariance.