Question

why C# can’t implicitly convert a long var to an object var then to ulong?

    long a = 0;
    Object c = a;
    ulong b = (ulong)c; // throw exception here

Answer 1

If you box a value type T, you can only unbox it as itself or as a Nullable ( T? ).
Any other cast is invalid.

That’s because a cast from object can never be interpreted as a conversion, whereas the is a conversion between long and ulong.

So this is legal:

var c = (long) b;

This is also legal:

var c = (long?) b;

But this is not:

var c = (ulong) b;

To do what you want to, you have to cast twice: the first is only unboxing, and the second is the actual conversion:

var c = (ulong)(long) b;

For further information, see this blog post by Eric Lippert.