Declare variable in one line if statement [duplicate]

Question

I wonder why this syntax is not allowed in c#?

var email = (User user = User.Current) != null ? user.Email : "[email protected]";

When this is allowed:

User user;
var email = (user = User.Current) != null ? user.Email : "[email protected]";

Why does the variable have to be defined already, and cannot be defined in the statement?

This would help to make lines shorter, for example:

var email = User.Current.Very.Complex.Path != null ? User.Current.Very.Complex.Path.Email : "[email protected]";

could be

var email = (var user = User.Current.Very.Complex.Path) != null ? user.Email : "[email protected]";

Quiteri

This is due to the scope/encapsulation. If you declare the variable in the ternary, it's only available within it and I suspect the compiler guesses this is a fault. Also, some times, having a few more lines is better than 1 if it improves readability. Personally, I'd rather this split over a few lines, but each to their own here! – Broke

Is () a scope? I thought this could work like a can be used in using(var a = GetVar()){ /*use a*/}. Ie: variables declared in one line if's can be used in the rest of the statement. – Quiteri

Please see this question – Kalynkam

@Pierre-LucPineault Ah, this seems like the answer! – Quiteri

IMHO doing that would decrease readability, why don't you want to just do the 2nd block? Your final example could just be turned in to User user; var email = (user = User.Current.Very.Complex.Path) != null ? user.Email : "[email protected]"; – Sausage

I can't see it at the moment, but it seems like the null-coalescing operator (??) should be able to help you out here. Maybe you need a default user instance somewhere. – Novel

@JoelCoehoorn Unfortunately not without having to constructing a new User: var email = (User.Current ?? new User { Email = "[email protected]" }).Email; – Quiteri

@ScottChamberlain, apparently the c# team disagrees. Declaration Expressions almost made it into c# 6.0 and will make it into a future release sooner or later. – Toucan

Answer 1

What you're trying to do is not allowed, per the C# specification. This is basically because a variable declaration/initialization is a statement, not an expression (see Why can't we define a variable inside an if statement? for more details; credit to Pierre-Luc Pineault for linking that).

Here is another way to do it, which I think has high readability and conciseness:

var user = User.Current.Very.Complex.Path;
var email = user != null ? user.Email : "[email protected]";

Barnaul