Why does the Linq Cast<> helper not work with the implicit cast operator?

Question

Please read to the end before deciding of voting as duplicate...

I have a type that implements an implicit cast operator to another type:

Answer 1

The short answer would be simply: the Cast method doesn't support custom conversion operators.

In the first example:

B b = a;
B b2 = (B)a;

the compiler can see this B(A a) operator during static analysis; the compiler interprets this as a static call to your custom operator method. In the second example:

foreach (object obj in source) 
    yield return (T)obj; 

that has no knowledge of the operator; this is implemented via unbox.any (which is the same as castclass if T is a ref-type).

There is also a third option: if you went via dynamic, the runtime implementation tries to mimic compiler rules, so this will find the operator ... but not as part of the C#-to-IL compile step:

dynamic b = a; // note that `dynamic` here is *almost* the same as `object`
B b2 = b;