Question

I ran into an issue today and I wasn’t entirely sure why it wouldn’t work.

The following code sample will crash:

static void Main(string[] args)
{
     int i32 = 10;
     object obj = i32;
     long i64 = (long)obj;
}

This will result in an InvalidCastException. Why does this not work? Is C# not smart enough to know that the object is actually of type int?

I’ve already come up with a workaround, but I’m curious as to why the above code sample didn’t work in the first place.

Thanks,
Tim

Answer 1

There is no cast available from a boxed Int32 to an Int64.
Making an intermediate cast to int should work, because the compiler is willing to generate this:

// verify obj is a boxed int, unbox it, and perform the *statically*
// known steps necessary to convert an int to a long
long i64 = (long) ((int)obj);

but not (hypothetically) this:

// Find out what type obj *actually* is at run-time and perform 
// the-known-only-at-run-time steps necessary to produce  
// a long from it, involving *type-specific* IL instructions  
long i64 = (long)obj; 

Here’s a blog post by Eric Lippert about this.