Why covariance and contravariance do not support value type

Question

IEnumerable is co-variant but it does not support value type, just only reference type. The below simple code is compiled successfully:

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Answer 1

It is perhaps easier to understand if you think about the underlying representation (even though this really is an implementation detail). Here is a collection of strings:

IEnumerable strings = new[] { "A", "B", "C" };

You can think of the strings as having the following representation:

[0] : string reference -> "A"
[1] : string reference -> "B"
[2] : string reference -> "C"

It is a collection of three elements, each being a reference to a string. You can cast this to a collection of objects:

IEnumerable