What is C#'s equivalent to Haskell's newtype?

Question

In Haskell, there's two ways of providing an alias for types: type and newtype. type provides a type synonym, which means the synonym is regarded by the type checker as exactly the same as the original type:

type UserId = Int
hasAccess :: UserId -> Bool
hasAccess id = {-- stuff --}

-- Elsewhere in the program
login :: Int -> Bool
login n = hasAccess n -- Typechecker won't complain

A newtype is similar, but is regarded by the type checker as a different type:

newtype UserId = UserId Int
hasAccess :: UserId -> Bool
hasAccess (UserId id) = {-- stuff --}

-- Elsewhere in the program
login :: Int -> Bool
login n = hasAccess n -- Typechecker will complain, n isn't a UserId !

In C#, you can define type synonyms with a top-level using declaration:

using UserId = Int;

However, a strongly-typed, compiler-checked type alias does not seem to be present in C# by default. I've looked into automatic code generation with T4 templates and CodeDOM to generate a class wrapper, but I don't really know how I could cleanly integrate those into my programming flow.

Ideally, I would like to be able to say on a top-level:

// Something like this?
using Int.UserId;

/* Elsewhere */
var id = new UserId(5);

public bool HasAccess( UserId id )
{
    /* Stuff */
}

This kicks the code generation into gear at compile-time. If that's not possible or provides a chicken-and-egg issue for IntelliSense, an automatic compilation option that runs every x minutes (or a button or whatever) would be nice.

Answer 1

To expand on Georg's answer, C# 10 has record structs which allows you to have:

public readonly record struct UserId(int Id);

Just a reminder that it's still possible to accidentally construct a UserId with the implicit parameterless constructor:

var uId = new UserId(); // Oops, forgot to pass the argument!

That's why you might want to catch the error at runtime by throwing:

public readonly record struct UserId(int Id)
{
    public UserId() : this(0)
    {
        throw new ArgumentException("Parameterless constructor should not be used");
    }
}

Even this won't prevent all mistakes because parameterless constructor is ignored in some cases.