Strange casting behaviour. Cannot cast object (int) to long

Question

I have the following code:

int intNumber1 = 100; object intNumber2 = 100; bool areNumberOfTheSameType = intNumber1.GetType() == intNumber2.GetType(); // TRUE bool areEqual = intNumber1.Equals(intNumber2); // TRUE  long longNumber1 = (long) intNumber1; // OK long longNumber2 = (long) intNumber2; // InvalidCastException. Why? 

Why doesn't the second cast work? I realize that it might be because the object doesn’t have an explicit cast to a long, but if we look at its type on runtime it is System.Int32.

If I use var or dynamic instead of object, it works.

Any thoughts?

Answer 1

Cast from int to long is interpreted as conversion between the two types.

Cast from object to int is interpreted as unboxing a boxed int.

It is the same syntax, but it says two different things.

In the working cases (int→long, object (boxed int)→int), the compiler knows exactly what code to produce. If boxed int→long was to work, the compiler would have to somehow figure out which conversion to use, but it doesn't have enough information to do it.

See also this blog post from Eric Lippert.

Answer 2

The object holds a type int. But it's considered an object (which is a boxed int) and a boxed value type can generally only be cast to its underlying type (the type that is boxed).

To cast it to another type, you first have to cast it to its underlying type. This works:

long longNumber2 = (long) (int) intNumber2; 

The reason that var works is that the compiler infers the type at compile time. That means, when you use var, the type of intNumber2 (if you use typeof) will be int. Whereas when you use object, the type will be object.

Using dynamic is a whole different process and cannot be compared with var. Here, the conversion / casting takes place at runtime, using reflection and the DLR library. It will dynamically find the underlying type, find that it has a conversion operator and uses that.