The difference between converting and unboxing: read my mind, compiler!

The Philosopher Developer

May 05, 2010

A developer on Stack Overflow was recently perplexed by the fact that the implicit operator he’d defined for his custom type didn’t seem to be working.

The scenario was this: the developer had a DataTable whose columns corresponded to the names of properties for a certain type of object. He had a bunch of PropertyInfo objects, one of which was of type Currency (his custom type with the implicit operator), and he was trying to read the table to get the value of each of his object’s properties. Obviously, since he couldn’t put a Currency object in the DataTable, he used decimal values instead, and defined an implicit conversion from decimal to Currency. Seems pretty smart, right?

Here’s the code he posted:

foreach (PropertyInfo p in props)
    p.SetValue(this, table.Rows[0][p.Name], null);

When it came to the Currency property, the program was throwing an ArgumentException. WTF? this developer thought. What happened to my implicit operator?

Well, here’s the first problem: the code above tries to pass a decimal to a call to SetValue for a PropertyInfo object whose PropertyType is Currency. So, presumably, the code could be fixed like this:

foreach (PropertyInfo p in props)
    Currency c = (Currency)table.Rows[0][p.Name];
    p.SetValue(this, c, null);

That should work, right?

Well, no. Here’s the thing: the implicit operator, as you may know, allows you to convert from one type to another without using an explicit cast. However, whenever you perform such a conversion, you’re still dealing with two fundamentally different types.

When a value type is boxed inside a System.Object, using that value requires first unboxing it – as its original type. Many C# developers are surprised to learn that the following code throws an exception:

int int32 = 0;
object boxedInt32 = int32;
long int64 = (long)boxedInt32;

Even though converting from int to long is totally doable, the compiler doesn’t know it’s supposed to do this if it’s just dealing with a System.Object.

To make this all a little more concrete, here’s an obnoxious little dialogue I just wrote – a dramatization of the above code snippet:

Programmer: Here is an int.
Compiler: Thank you!
Programmer: Now please put that int in a box.
Compiler: OK! Done!
Programmer: Now, take a long out of that box.
Compiler: OK–wait, WTF?

And here’s a similar dialogue depicting what happened with the Stack Overflow user’s code:

Programmer: Hey, Compiler, I’m sending you a box.
Compiler: Oh boy! I wonder what’s in it?
Programmer: I’ll give you a hint! It starts with “C” and rhymes with “urrency.”
Compiler: A Currency! Sweet!
Programmer: Yep, that’s right! Now here it is…
Compiler (opening box): Yesss, a Cur—wait a second.
Programmer: What’s wrong, Compiler?
Compiler: You said this was going to be a Currency. It’s clearly a decimal.
Programmer: So what? Aren’t they the same thing to you?
Compiler: No. They’re not. Are they the same to you? Geez. If you had told me this was a decimal, I could’ve brought my tools to convert it to a Currency, but…
Programmer: Come on, why didn’t you just know it was a decimal?
Compiler: Screw you.

Moral of the story: even though the syntax happens to be the same, unboxing a value and converting from one type to another are two different operations.

Oh yeah, and here’s the code that actually does work:

foreach (PropertyInfo p in props)
     if (p.PropertyType == typeof(Currency))
         // unbox, THEN use implicit operator
         Currency c = (decimal)table.Rows[0][p.Name];
         p.SetValue(this, c, null);
         p.SetValue(this, table.Rows[0][p.Name], null);