Null coalescing operator (??) with return

Question

I was wondering why it is possible to do this in C# 7.0:

int? test = 0;
int test2 = test ?? throw new Exception("Error");

..but not this:

int? test = 0;
int test2 = test ?? return;

Can someone explain that?

Answer 1

throw has relatively recently (in C# 7.0) been turned into an expression to enable this. return hasn't - it is always a just statement. The ?? operator requires an expression, not a statement. It was an arbitrary choice by the C# designers, specifically to allow using throw with ??.

See the documentation on the throw expression

Answer 2

Well, because no one has implemented it this way. Or, more precisely, because return is not an expression.

throw used to be a statement only prior to C# 7.0, but then was extended (due to a proposal) to be an expression as well (only expressions are supported in the null coalescing operator).

So unless no one suggests to make return an expression, it won't work.