Official proposal for Type Unions in C#(github.com)

  Q: Why are there no tagged unions?
  A: Union structs are both tagged unions and type unions. Under the hood, a union struct is a tagged union, even exposing an enum property that is the tag to enable faster compiler generated code, but in the language it is presented as a type union to allow you to interact with it in familiar ways, like type tests, casts and pattern matching.

  // Object-Oriented Approach
  interface Shape {
      fun area(): Double
      fun perimeter(): Double
  }

  class Circle(val radius: Double) : Shape {
      override fun area() = Math.PI \* radius \* radius
      override fun perimeter() = 2 \* Math.PI \* radius
  }

  class Rectangle(val width: Double, val height: Double) : Shape {
      override fun area() = width \* height
      override fun perimeter() = 2 \* (width + height)
  }

  // Easy to add new shape
  class Triangle(val a: Double, val b: Double, val c: Double) : Shape {
      override fun area(): Double {
          val s = (a + b + c) / 2
          return Math.sqrt(s \* (s - a) \* (s - b) \* (s - c))
      }
      override fun perimeter() = a + b + c
  }

  // Hard to add new function (need to modify all existing shapes)
  // interface Shape {
  //     fun area(): Double
  //     fun perimeter(): Double
  //     fun draw(): String  // New function
  // }

  // Functional Approach
  sealed class ShapeFP {
      data class CircleFP(val radius: Double) : ShapeFP()
      data class RectangleFP(val width: Double, val height: Double) : ShapeFP()
  }

  fun area(shape: ShapeFP): Double = when (shape) {
      is ShapeFP.CircleFP -> Math.PI \* shape.radius \* shape.radius
      is ShapeFP.RectangleFP -> shape.width \* shape.height
  }

  fun perimeter(shape: ShapeFP): Double = when (shape) {
      is ShapeFP.CircleFP -> 2 \* Math.PI \* shape.radius
      is ShapeFP.RectangleFP -> 2 \* (shape.width + shape.height)
  }

  // Easy to add new function
  fun draw(shape: ShapeFP): String = when (shape) {
      is ShapeFP.CircleFP -> "O"
      is ShapeFP.RectangleFP -> "[]"
  }

  // Hard to add new shape (need to update all existing functions)
  // sealed class ShapeFP {
  //     data class CircleFP(val radius: Double) : ShapeFP()
  //     data class RectangleFP(val width: Double, val height: Double) : ShapeFP()
  //     data class TriangleFP(val a: Double, val b: Double, val c: Double) : ShapeFP()
  // }

  using System.Runtime.InteropServices;
  var s = new S { Bar = "Baz" };
  [StructLayout(LayoutKind.Explicit)]
  public struct S {
      [FieldOffset(0)] public UInt64 Foo;
      [FieldOffset(0)] public Object Bar;
  }

    public abstract record Maybe<A>
    {
        private Maybe() { }

        public sealed record Just(A Value) : Maybe<A>;
        public sealed record Nothing : Maybe<A>;
    }

    var mx = new Maybe<int>.Just(123);
    var my = new Maybe<int>.Nothing();

   var r = mx switch
   {
      Maybe<int>.Just (var x) => x,
      Maybe<int>.Nothing      => 0
   };

   if(option.IsT1){ doSomething(option.ValueT1)};
   if(option.IsT1){doSomething(option.ValueT2)}; // this would throw an exception, i.e. a bug, but its very readable to catch the bug