How to add using the 'with' keyword?

Question

public record Cube(int x, int y, int z, int w);

I recently ran across a time I was doing this:

var next = new Cube(existing.x + 1, existing.y, existing.z, existing.w);

and I thought there must be a cleaner way to add arbitrary values to the record. But when I try this:

var next = existing with { x = x+1 };

It cries because you do not have access to the values to add to. Instead I have to do this:

var next = existing with { x = existing.x+1 };

Am I just wanting too much from the with keyword?

jjxtra_56079 · Answer

My understanding of with syntax is exactly that - you must reference the name of the object to the left of the width statement. This seems to line up with the spec here: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/records

While the x = x + 1 is convenient, it is not very expressive on the source of where x is coming from.

user_13439267 · Answer

The x in x = x+1 is the x of the next instance to be created. However, this instance does not yet exist at that point that's why the compiler complains. Instead, you have to state explicitly that the second x is the one of the existing instance.

How to add using the 'with' keyword?

2 Answers