F# algebraic types under the covers

Consider the simple type

	// After the Haskell type
	type internal Either<'a, 'b> =
	| Left of 'a
	| Right of 'b

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Innocuous, right? Well look at what Reflector has to say about the May 2009 CTP version...

	[Serializable, DebuggerDisplay("{__DebugDisplay()}"), CompilationMapping(SourceConstructFlags.NonpublicRepresentation | SourceConstructFlags.SumType)]
	internal abstract class Either<a, b> : IStructuralEquatable, IComparable, IStructuralComparable
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal const int tag_Left = 0;
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal const int tag_Right = 1;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	internal protected Either();
	internal object __DebugDisplay();
	[CompilerGenerated]
	internal int CompareTo(Either<a, b> obj);
	[CompilerGenerated]
	public sealed override int CompareTo(object obj);
	[CompilerGenerated]
	public sealed override int CompareTo(object obj, IComparer comp);
	[CompilerGenerated]
	public sealed override bool Equals(object obj);
	[CompilerGenerated]
	internal bool Equals(Either<a, b> obj);
	[CompilerGenerated]
	public sealed override bool Equals(object obj, IEqualityComparer comp);
	[CompilerGenerated, DebuggerNonUserCode]
	public override int GetHashCode();
	[CompilerGenerated]
	public sealed override int GetHashCode(IEqualityComparer comp);
	[CompilerGenerated, DebuggerNonUserCode]
	internal bool IsLeft();
	[CompilerGenerated, DebuggerNonUserCode]
	internal bool IsRight();
	[CompilationMapping(SourceConstructFlags.UnionCase, 0)]
	internal static Either<a, b> Left(a Left1);
	[CompilationMapping(SourceConstructFlags.UnionCase, 1)]
	internal static Either<a, b> Right(b Right1);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 0, 0), CompilerGenerated, DebuggerNonUserCode]
	internal a Left1 { [CompilerGenerated, DebuggerNonUserCode] get; }
	[CompilationMapping(SourceConstructFlags.Field, 1, 0), CompilerGenerated, DebuggerNonUserCode]
	internal b Right1 { [CompilerGenerated, DebuggerNonUserCode] get; }
	[CompilerGenerated, DebuggerNonUserCode, DebuggerBrowsable(DebuggerBrowsableState.Never)]
	internal int Tag { [CompilerGenerated, DebuggerNonUserCode] get; }
	// Nested Types
	[Serializable, DebuggerTypeProxy(typeof(Either<a, b>._Left@DebugTypeProxy<,>)), DebuggerDisplay("{__DebugDisplay()}")]
	internal class _Left : Either<a, b>
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal readonly a left1;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	internal _Left(a left1);
	}
	private class _Left@DebugTypeProxy
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	private Either<a, b>._Left _obj;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	public _Left@DebugTypeProxy(Either<a, b>._Left obj);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 0, 0), CompilerGenerated, DebuggerNonUserCode]
	public a Left1 { [CompilerGenerated, DebuggerNonUserCode] get; }
	}
	[Serializable, DebuggerTypeProxy(typeof(Either<a, b>._Right@DebugTypeProxy<,>)), DebuggerDisplay("{__DebugDisplay()}")]
	internal class _Right : Either<a, b>
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal readonly b right1;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	internal _Right(b right1);
	}
	private class _Right@DebugTypeProxy
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	private Either<a, b>._Right _obj;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	public _Right@DebugTypeProxy(Either<a, b>._Right obj);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 1, 0), CompilerGenerated, DebuggerNonUserCode]
	public b Right1 { [CompilerGenerated, DebuggerNonUserCode] get; }
	}
	}

view raw gistfile1.cs hosted with ❤ by GitHub

Most of this is tagged as [CompilerGenerated] -- with some surprising omissions, like

	internal object __DebugDisplay();
	[CompilationMapping(SourceConstructFlags.UnionCase, 0)]
	internal static Either<a, b> Left(a Left1);
	[CompilationMapping(SourceConstructFlags.UnionCase, 1)]
	internal static Either<a, b> Right(b Right1);

view raw gistfile1.cs hosted with ❤ by GitHub

which makes filtering out what is written vs what is generated for coverage analysis purposes rather more tedious than it might be. Arguably, in a unit test using the type, the latter two should get exercised anyway, but having the __DebugDisplay() method left over seems like a bit of an oversight.

---

By way of comparison, the Feb 2010 CTP yields

	[Serializable, DebuggerDisplay("{__DebugDisplay(),nq}"), CompilationMapping(SourceConstructFlags.NonPublicRepresentation | SourceConstructFlags.SumType)]
	internal abstract class Either<a, b> : IEquatable<Either<a, b>>, IStructuralEquatable, IComparable<Either<a, b>>, IComparable, IStructuralComparable
	{
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	internal Either();
	[CompilerGenerated, DebuggerNonUserCode]
	internal object __DebugDisplay();
	[CompilerGenerated]
	public sealed override int CompareTo(object obj);
	[CompilerGenerated]
	public sealed override int CompareTo(Either<a, b> obj);
	[CompilerGenerated]
	public sealed override int CompareTo(object obj, IComparer comp);
	[CompilerGenerated]
	public sealed override bool Equals(object obj);
	[CompilerGenerated]
	public sealed override bool Equals(Either<a, b> obj);
	[CompilerGenerated]
	public sealed override bool Equals(object obj, IEqualityComparer comp);
	[CompilerGenerated]
	public sealed override int GetHashCode();
	[CompilerGenerated]
	public sealed override int GetHashCode(IEqualityComparer comp);
	[CompilationMapping(SourceConstructFlags.UnionCase, 0)]
	internal static Either<a, b> NewLeft(a item);
	[CompilationMapping(SourceConstructFlags.UnionCase, 1)]
	internal static Either<a, b> NewRight(b item);
	// Properties
	[CompilerGenerated, DebuggerNonUserCode, DebuggerBrowsable(DebuggerBrowsableState.Never)]
	internal bool IsLeft { [CompilerGenerated, DebuggerNonUserCode] get; }
	[CompilerGenerated, DebuggerNonUserCode, DebuggerBrowsable(DebuggerBrowsableState.Never)]
	internal bool IsRight { [CompilerGenerated, DebuggerNonUserCode] get; }
	[CompilerGenerated, DebuggerNonUserCode, DebuggerBrowsable(DebuggerBrowsableState.Never)]
	internal int Tag { [CompilerGenerated, DebuggerNonUserCode] get; }
	// Nested Types
	[Serializable, DebuggerTypeProxy(typeof(Either<a, b>.Left@DebugTypeProxy<,>)), DebuggerDisplay("{__DebugDisplay(),nq}")]
	internal class Left : Either<a, b>
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal readonly a item;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	internal Left(a item);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 0, 0), CompilerGenerated, DebuggerNonUserCode]
	internal a Item { [CompilerGenerated, DebuggerNonUserCode] get; }
	}
	internal class Left@DebugTypeProxy
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal Either<a, b>.Left _obj;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	public Left@DebugTypeProxy(Either<a, b>.Left obj);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 0, 0), CompilerGenerated, DebuggerNonUserCode]
	public a Item { [CompilerGenerated, DebuggerNonUserCode] get; }
	}
	[Serializable, DebuggerTypeProxy(typeof(Either<a, b>.Right@DebugTypeProxy<,>)), DebuggerDisplay("{__DebugDisplay(),nq}")]
	internal class Right : Either<a, b>
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal readonly b item;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	internal Right(b item);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 1, 0), CompilerGenerated, DebuggerNonUserCode]
	internal b Item { [CompilerGenerated, DebuggerNonUserCode] get; }
	}
	internal class Right@DebugTypeProxy
	{
	// Fields
	[DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode]
	internal Either<a, b>.Right _obj;
	// Methods
	[CompilerGenerated, DebuggerNonUserCode]
	public Right@DebugTypeProxy(Either<a, b>.Right obj);
	// Properties
	[CompilationMapping(SourceConstructFlags.Field, 1, 0), CompilerGenerated, DebuggerNonUserCode]
	public b Item { [CompilerGenerated, DebuggerNonUserCode] get; }
	}
	internal static class Tags
	{
	// Fields
	public const int Left = 0;
	public const int Right = 1;
	}
	}

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which is broadly similar, but differs in detail (such as the Tags values and the DebuggerDisplay attributes -- and the bug about __DebugDisplay not having the CompilerGenerated attribute is fixed.