C#•14mo ago

❔ Using `with` to create a new record in return statement result in an error

    private Point GetNewPosition(Commands command, Point position)
    {
        if (command == Commands.MoveNorth) return position with { Y++ };
        if (command == Commands.MoveSouth) return position with { Y-- }; 
        if (command == Commands.MoveEast) return position with { X++ };
        if (command == Commands.MoveWest) return position with { X-- };
        return null;
    }
  public record Point(int X, int Y);

    private Point GetNewPosition(Commands command, Point position)
    {
        if (command == Commands.MoveNorth) return position with { Y++ };
        if (command == Commands.MoveSouth) return position with { Y-- }; 
        if (command == Commands.MoveEast) return position with { X++ };
        if (command == Commands.MoveWest) return position with { X-- };
        return null;
    }
  public record Point(int X, int Y);

I'm trying to make new records using with statement, but it resulted in an error saying The name 'Y' does not exist in the current context Can anybody help explain? Thanks

28 Replies

Angius•14mo ago

Try with { Y = Y + 1 } and so on?

EddieV•14mo ago

Did it, still get the same error

Pobiega•14mo ago

ah, you can't do Y++ like that you need to do return position with { Y = position.Y + 1 }; also, this could (and should) be a switch expression

private Point? GetNewPosition(Commands command, Point position) =>
    command switch
    {
        Commands.MoveNorth => position with { Y = position.Y + 1 },
        Commands.MoveSouth => position with { Y = position.Y - 1 },
        Commands.MoveEast => position with { X = position.X + 1 },
        Commands.MoveWest => position with { X = position.X - 1 },
        _ => null
    };

private Point? GetNewPosition(Commands command, Point position) =>
    command switch
    {
        Commands.MoveNorth => position with { Y = position.Y + 1 },
        Commands.MoveSouth => position with { Y = position.Y - 1 },
        Commands.MoveEast => position with { X = position.X + 1 },
        Commands.MoveWest => position with { X = position.X - 1 },
        _ => null
    };

Point? is the correct returntype, since it might return null.

EddieV•14mo ago

Thanks @Pobiega . I changed the code as you suggested and it indeed worked!! And the switch statement looks so much nicer than my ifs 😂 Would you mind explaining why I couldn't use Y++ there?

Pobiega•14mo ago

its just not possible to use increment decrement operators with with sadly its worth remembering that Y++ does two things - it both evaluates into a number, but also mutates the number thats probably why

EddieV•14mo ago

I see. Thank you. Just one more unrelated question about this

Point? is the correct returntype, since it might return null.

Pobiega•14mo ago

nullable reference types ❤️

EddieV•14mo ago

Does it mean I should always put ? if any of my methods might return null?

Pobiega•14mo ago

yep. in fact, what .net version are you using?

EddieV•14mo ago

6.0.402

Pobiega•14mo ago

its a language feature added in .NET 5, and since 6.0 it defaults to enabled

EddieV•14mo ago

Ah, so it's not necessary for .NET >= 6?

Pobiega•14mo ago

erm, rather, Im suprised your IDE isnt yelling at you

EddieV•14mo ago

I don't know 😂 I'm coming from Python and C# is still fairly new to me! Anyway, thanks for your help. Really appreciate it! 😄

Pobiega•14mo ago

can read this if you want to know more https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-reference-types

Nullable reference types - C# reference

Learn about C# nullable reference types and how to use them

Pobiega•14mo ago

the idea is that any reference type can ofc be null but that ability often causes issues during runtime its annoying having to nullcheck things in every single method so the idea was, "what if we add a way to document if a method can or can't return null" consider these two methods

private Point? GetNewPosition(Commands command, Point position) =>
    command switch
    {
        Commands.MoveNorth => position with { Y = position.Y + 1 },
        Commands.MoveSouth => position with { Y = position.Y - 1 },
        Commands.MoveEast => position with { X = position.X + 1 },
        Commands.MoveWest => position with { X = position.X - 1 },
        _ => null
    };


private Point GetNewPosition(Commands command, Point position) =>
    command switch
    {
        Commands.MoveNorth => position with { Y = position.Y + 1 },
        Commands.MoveSouth => position with { Y = position.Y - 1 },
        Commands.MoveEast => position with { X = position.X + 1 },
        Commands.MoveWest => position with { X = position.X - 1 },
        _ => position
    };

private Point? GetNewPosition(Commands command, Point position) =>
    command switch
    {
        Commands.MoveNorth => position with { Y = position.Y + 1 },
        Commands.MoveSouth => position with { Y = position.Y - 1 },
        Commands.MoveEast => position with { X = position.X + 1 },
        Commands.MoveWest => position with { X = position.X - 1 },
        _ => null
    };


private Point GetNewPosition(Commands command, Point position) =>
    command switch
    {
        Commands.MoveNorth => position with { Y = position.Y + 1 },
        Commands.MoveSouth => position with { Y = position.Y - 1 },
        Commands.MoveEast => position with { X = position.X + 1 },
        Commands.MoveWest => position with { X = position.X - 1 },
        _ => position
    };

the bottom one can never return null but the top one can, so we add a ? to the return type to document that this will give us warnings if we try to use that value without checking it for null to prevent NullRefExceptions at runtime 🙂

EddieV•14mo ago

Let me read the documentation a bit more Thank you for the useful info

Pobiega•14mo ago

EddieV•14mo ago

Now I noticed it, the IDE did yell at me via these lines

EddieV•14mo ago

I tried to put ? to where appropriate and the warnings left one by one except here return new FountainOfObjects(name); and here SendMessage(message); my SendMessage returns void

Pobiega•14mo ago

yeah but message might be null, and SendMessage probably doesnt take message? so it wants you to make sure message is not null before passing it into SendMessage

EddieV•14mo ago

I think I did? string? message = _world.GetMessage(_player.Position);

Pobiega•14mo ago

well, not really you've stored it in a variable that says "this might be null" you have not actually CHECKED if it is null or not

EddieV•14mo ago

Ah I see I checked for null within the SendMessage method instead of checking before sending it to the method

Pobiega•14mo ago

thats fine, but then your method could either accept string? or you move the check out 🙂

EddieV•14mo ago

Yeah, I moved the check out and managed to get rid off the warning of null at the method to instantiate a new object too. You're right, it could get a bit of annoying 😂 No more yellowy warning!! Thanks very much for your help

Pobiega•14mo ago

Accord•14mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Using `with` to create a new record in return statement result in an error