Const ValueTuple

[ghost]

Why can't ValueTuple's be declared const? I understand why value types generally can't be declared const but the reason about constructor can't be aplied to ValueTuples.

I tried to use const (int, int) tuple = (1, 2); in my code but it's not allowed.

[Joe4evr]

Because there's no primitive representation of the ValueTuple types in the CLR.

Also, the ValueTuple types are mutable, so that's a whole other can of worms.

[iam3yal]

@drozdekl There are at least three issues with this:

1. Constants require to be initialized at the point of declaration.

2. You can't declare a field of a generic type as a constant even when the constrain is struct.

3. Because of the above points you can't have a primitive representation such as ConstValueTuple as @Joe4evr pointed out.

So the following would work:

void Main()
{
	new ConstValueTuple<int, int>();
}

struct ConstValueTuple<T1, T2> where T1: struct where T2: struct
{
	public ConstValueTuple(T1 item1, T2 item2)
	{
	}
}

But the following isn't possible:

struct ConstValueTuple<T1, T2> where T1: struct where T2: struct
{
	public ConstValueTuple(T1 item1, T2 item2)
	{
		Item1 = item1;
		
		Item2 = item2;
	}
	
	public const T1 Item1; 
	
	public const T2 Item2;
}

[ghost]

@eyalsk Thank you for your answer. I thought that it would be great to have const tuples but I understand that it is not possible. I thought that all necessary things would be known at compile time. If one wrote const (int,int) tuple = (1,2) or even const ((int,int),int) tuple = ((1,2),3) then there are const values on the right side and ValueTuples are so simple that I would expect this would be OK. Anyway, thank you!

I almost forgot that ValueTuples are not built in and I shoudn't expect anything special about them.

[iam3yal]

@drozdekl You welcome. :)

[ghost]

@eyalsk

I would like to ask another question but do not want to start a thread. Why can't ValueTuples consist only from one value? I have written buffered reader which knows key value from last reading and would like to make return value from writer in the same way.

Currently I have:

public interface IReaderStream<T> : IDisposable {
	(T Item, object LastKey) Read(object key);
}

interface IWriterStream<T> : IDisposable {
	object Write(T item, object key);
}

public interface IReader<T> {
	T ReadFrom(IReaderStream<T> source);
}

interface IWriter<T> {
	T WriteTo(IReaderStream<T> destination);
}

But I would like to write (object LastKey) Write(T item, object key);

[iam3yal]

@drozdekl You can check #883. :)

[ghost]

@eyalsk

Is there any possibility in language to have an interface which is publicly sealed? I would like to have such interface which would provide an interface to user and could be anytime completely replaced. Also I would like to make sure that I control which classes can implement that interface to influence correct behaviour.

public static class Provider {
    public IItem New() => new ItemImplementation();
}

// publicly sealed
public internal IItem {

}

internal class ItemImplementation() {

}

User of that dll could write:

IItem x = Provider.Build();

but not

class A : IItem {
}

[svick]

@drozdekl There is an old proposal to do that: dotnet/roslyn#220. It's not clear to me why it was closed. There's also another similar (but broader) proposal at #485.

---

But I don't think you should ask questions not related to your original issue here. You should either open a new issue or move to some other place, like the gitter channel (@eyalsk seems to be present there).