Question

Consider the following code:

class Program
{
    public static explicit operator long(Program x) { return 47; }

    static int Main(string[] args)
    {
        var x = new Program();
        Console.WriteLine((decimal) x);
    }
}

To my surprise, this outputs 47; in other words, the explicit operator long is called even though the cast is to decimal.

Is there something in the C# spec that explicitly says that this should happen (if so, where exactly) or is this the result of some other rule(s) I’m missing?

Answer 1

I’ve found the answer. First of all, there is the concept of one type being encompassed by another, which is defined in 6.4.3 Evaluation of user-defined conversions as follows:

If a standard implicit conversion (§6.3.1) exists from a type A to a
type B, and if neither A nor B are interface-types, then A is said to
be encompassed by B, and B is said to encompass A.

6.3.1 Standard implicit conversions states that “Implicit numeric conversions (§6.1.2)” are a standard implicit conversion, and 6.1.2 Implicit numeric conversions in turn defines an implicit conversion from long to decimal. Therefore, long is encompassed by decimal.

Next, 6.4.5 User-defined explicit conversions states that one of the stages in determining whether an explicit conversion is applicable is to:

Find the set of applicable user-defined and lifted conversion
operators, U. This set consists of the user-defined and lifted
implicit or explicit conversion operators declared by the classes or
structs in D that convert from a type encompassing or encompassed by S
to a type encompassing or encompassed by T. If U is empty, the
conversion is undefined and a compile-time error occurs.

Here, D refers to the result of an earlier step which, in this case, contains only decimal, Program and object. Thus, the set U will contain the Program-to-long explicit operator I declared because long is encompassed by decimal (as we found earlier).

One of the next steps selects long as the most specific target type, TX.

Finally, the last step in that same algorithm states:

Finally, apply the conversion:

• If S is not SX, then a standard explicit conversion from S to SX is performed.
• The most specific user-defined conversion operator is invoked to convert from SX to TX.
• If TX is not T, then a standard explicit conversion from TX to T is performed.

Here, S and SX are both Program, so the first part does nothing. TX was selected to be long and T is the target type, decimal, so the last part executes the standard conversion from long to decimal.