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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Loyc
/// <summary><c>@void.Value</c> represents the sole value of <c>System.Void</c>
/// (called "void" in C#).</summary>
/// <remarks>.NET unfortunately treats void as something that is not a real
/// type; for example you cannot use <c>new void()</c> or <c>default(void)</c>
/// in C#. This was a dumb decision because it means that some generic code
/// must be duplicated for void and non-void types. A good example is the fact
/// that when you have a <see cref="Dictionary{TKey,TVal}"/>, TVal cannot be
/// void, so you cannot use <c>Dictionary(string,void)</c> to express the idea
/// of "a set of strings with no associated values". The <see cref="HashSet{T}"/>
/// class uses a completely separate implementation and cannot just be an
/// alias for <c>Dictionary{T,void}</c> (actually they could share
/// implementations using a dummy type like this one, but unfortunately .NET
/// made another dumb decision that all types must consume at least one byte,
/// so <c>HashSet</c> sharing code with <c>Dictionary</c> would make
/// <c>HashSet</c> less efficient.)
/// <para/>
/// Defining a @void type allows you to use it when it makes conceptual sense,
/// although we cannot avoid .NET's requirement to waste at least one byte.
/// </remarks>
public struct @void
public static readonly @void Value = new @void();
public override string ToString()
return "void";
/// <summary><c>NoValue.Value</c> is meant to be used as the value of a
/// property that has "no value", meaning no value is assigned or that the
/// property is meaningless at the current time or in the current context.
/// </summary><remarks>
/// Most often <c>null</c> is used for this purpose; <c>NoValue.Value</c>
/// is used when <c>null</c> is a considered to be a valid, meaningful
/// value and you want to distinguish between "no value" and "null".
/// For example, this is used by the the Value property of Loyc.Syntax.LNode,
/// in which <c>NoValue</c> has a different meaning than both <c>null</c>
/// and <c>void</c>.
/// <para/>
/// Also, this value converts implicitly to <see cref="Maybe{T}.NoValue"/>.
/// </remarks>
public class NoValue
private NoValue() { }
public static readonly NoValue Value = new NoValue();
public override string ToString()
return "(No value)".Localized();