C# tuple field names are an interesting case, spiritually similar to this proposal's ad hoc unions: persisted in compiled output via attributes, respected by the compiler even across assembly boundaries, but not actually part of the resulting CLR type. So, e.g., if

  var p = (x: 1, y: 2);
  var q = (u: 1, y: 2);
  var f = ((int x, int y) a) => a.x + a.y;

then

  p == q 
  f(p) == 3
  f(q) == 3
  p.GetType() == q.GetType()

are all true, but

  var r = q.x;

yields the compile-time error

CS1061: '(int u, int v)' does not contain a definition for 'x' and no accessible extension method 'x' accepting a first argument of type '(int u, int v)' could be found (are you missing a using directive or an assembly reference?)

and if

  dynamic d = q;

then

  var s = d.u;

compiles, but throws

RuntimeBinderException: 'System.ValueTuple<int,int>' does not contain a definition for 'u'

at runtime, revealing the underlying CLR type.

> tuples are "untagged", records/structs are "tagged" (the members have names).

Not always true in C#. Tuples can have member names

https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...