Question

I have InvalidCastException when I try to cast 0.0 to double, why is that so? It’s fine when I do (float)value instead.

Answer 1

In general, when you put a value type into an object (called boxing) you need to unbox it to the exact same value type. You cannot do a conversion to another type instead. This is what happens here.

If you really want to convert the object, you first need to unbox it. Say your original value was a float before you boxed it in an object:

double d = (double) (float) value;

Or use the method proposed by others, which uses Convert. This has the advantage that the original type doesn’t have to be known.