Casting null doesn't compile

Question

accidentally at work I wrote the following line of code:

string x = (object) null; 
// It was var x = (object)null and I changed from var to string instead of 
// object x = null;

This gave me a compilation error similar to this: Can't cast source type object to target type string

Why? Isn't null just a bunch of zeros pointing to "nowhere" memory address, no matter what the type is?

Answer 1

The problem is not the casting of null, it's that object isn't assignable to string. This works fine

string x = (string)null;

The reason this works if you remove the cast (string x = null) is laid out in section 2.4.4.6 of the C# Language Specification

The null-literal can be implicitly converted to a reference type or nullable type

The moment you introduce a cast ((object)null) you no longer have a null literal. Instead you have a value of type object. It's essentially no different than

object temp = null;
string x = temp;