场景
枚举，其实就是一种特殊的整数，只不过只能取一系列特定的值。通过 enum Type : short 这样的语法，我们可以指定枚举在底层究竟使用哪种整数。
然而，有的时候我们希望自定义类型，其实例有着各种我们所需要的成员；但同时我们又希望这个类型只有有限个实例，用户只能从其中取一个使用。比如，Anders Liu有一个系统，能够处理Word、Html和Text格式的文档，现在我希望定义一个DocumentType类型，表示所有支持的文档类型；但对于每一种类型，Anders Liu又偏偏希望它具有诸如Processor（处理这种类型文档的程序）和Extension（这种类型的文档的扩展名）等属性。此时就出现了矛盾。如果使用枚举，必然无法实现这些实例属性。但是，如果自己编写一个类呢？在这篇文章中，Anders Liu将带你一起实现一个用起来像枚举，但实际上是自定义类型的类。我要的对象，它不是整数首先，毕竟我们所需要的是对象，而不是枚举所提供的简单整数。所以，先定义好再说：
namespace AndersLiu.Samples.EnumSimilarClass
{ 
/**//// <summary>
/// Indicates the surpported document types. /// You cannot inherits from this class. /// </summary> /// <remarks> /// This class looks very like an enum.
/// </remarks> public sealed class DocumentType { instanse members#region instanse members string _name; string _processor; string _extension; /**//// <summary> /// Initialize a <see cref="DocumentType"/> instance. /// This constructor is private, so that user cannot construct it from external. /// </summary> /// <param name="name">Name of the document type.</param> /// <param name="processor">Processor of the document.</param> /// <param name="extension">Extension of the document.</param> private DocumentType(string name, string processor, string extension) { _name = name; _processor = processor; _extension = extension; } /**//// <summary> /// Name of the document type. /// </summary> public string Name { get { return _name; } } /**//// <summary> /// Processor of the document. /// </summary> public string Processor { get { return _processor; } } /**//// <summary> /// Extension of the document. /// </summary> public string Extension { get { return _extension; } } #endregion }
} foundation supports #region foundation supports static Dictionary<string, DocumentType> _allTypes; // Indicates the documents' type name. const string WordTypeName = "Word"; const string HtmlTypeName = "Html"; const string TextTypeName = "Text"; // Indicates which application can be used to open the document. const string WordProcessor = "winword.exe"; const string HtmlProcessor = "iexplorer.exe"; const string TextProcessor = "notepad.exe"; // Indicates the file extension of each document type. const string WordExtension = ".doc"; const string HtmlExtension = ".html"; const string TextExtension = ".txt"; /**//// <summary> /// Static constructor. Initializes all supported document types. /// </summary> static DocumentType() { _allTypes = new Dictionary<string, DocumentType>(); _allTypes.Add(WordTypeName, new DocumentType(WordTypeName, WordProcessor, WordExtension)); _allTypes.Add(HtmlTypeName, new DocumentType(HtmlTypeName, HtmlProcessor, HtmlExtension)); _allTypes.Add(TextTypeName, new DocumentType(TextTypeName, TextProcessor, TextExtension)); } #endregion enum similar members #region enum similar members /**//// <summary> /// Word document. /// </summary> public static DocumentType Word { get { return _allTypes[WordTypeName]; } } /**//// <summary> /// Html document. /// </summary> public static DocumentType Html { get { return _allTypes[HtmlTypeName]; } } /**//// <summary> /// Text document. /// </summary> public static DocumentType Text { get { return _allTypes[TextTypeName]; } } #endregion type convert oeprators #region type convert oeprators /**//// <summary> /// Implicit convert <see cref="DocumentType"/> object to <see cref="string"/>. /// </summary> /// <param name="type">A given document type.</param> /// <returns>The type name.</returns> public static implicit operator string(DocumentType type) { return type.Name; } /**//// <summary> /// Explicit convert <see cref="string"/> object to <see cref="DocumentType"/>. /// </summary> /// <param name="typeName">Given document type name.</param> /// <returns>The corresponsive document type.</returns> public static explicit operator DocumentType(string typeName) { if(_allTypes.ContainsKey(typeName)) return _allTypes[typeName]; else throw new InvalidOperationException(string.Format("'{0}' is not a valid document type.", typeName)); } #endregion other functions #region other functions /**//// <summary> /// Indicates a given type name is valid or not. /// </summary> /// <param name="typeName">Given type name.</param> /// <returns>Whether the <see cref="typeName"/> is vliad.</returns> public static bool IsValidDocumetTypeName(string typeName) { return _allTypes.ContainsKey(typeName); } /**//// <summary> /// Override. Generate the string representation of the current object. /// </summary> /// <returns>The string representation.</returns> public override string ToString() { return string.Format( "{0} (Processor=[{1}], Extension=[{2}])", _name, _processor, _extension); } #endregion /**/ /// <summary> /// Process a document with given type. /// </summary> /// <param name="doc">Type of the document.</param> private void process(DocumentType doc) { string msg = string.Format( "I'm working with a {0} document in {1}.", (string)doc, doc.Processor); // Look, use it just like a normal object. MessageBox.Show(msg); } private void btnProcessWord_Click( object sender, EventArgs e) { process(DocumentType.Word); // Look, it looks similarly an enum! } private void btnProcessHtml_Click( object sender, EventArgs e) { process(DocumentType.Html); } private void btnProcessText_Click( object sender, EventArgs e) { process(DocumentType.Text); }