The results of Listing 77 appear in Output 72 in Visual C#.NET

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The results of Listing 77 appear in Output 72
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OUTPUT 72:
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ISettingsProviderGetSetting in explicit interface declaration is not a member of interface
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This output appears in addition to an error indicating that IReadableSettingsProviderGetSetting() is not implemented The fully qualified
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interface member name used for explicit interface member implementation must reference the interface name in which it was originally declared Even though a class implements an interface (ISettingsProvider) that is derived from a base interface (IReadableSettingsProvider), the class can still declare implementation of both interfaces overtly, as Listing 78 demonstrates
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Listing 78: Using a Base Interface in the Class Declaration
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class FileSettingsProvider : ISettingsProvider, IReadableSettingsProvider { #region ISettingsProvider Members public void SetSetting(string name, string value) { // } #endregion #region IReadableSettingsProvider Members public string GetSetting(string name, string defaultValue) { // } #endregion }
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7: Interfaces
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In this listing, there is no change to the interface s implementations on the class, and although the additional interface implementation declaration on the class header is superfluous, it can provide better readability The decision to provide multiple interfaces rather than just one combined interface depends largely on what the interface designer wants to require of the implementing class By providing an IReadableSettingsProvider interface, the designer communicates that implementers are required only to implement a settings provider that retrieves settings They do not have to be able to write to those settings This reduces the implementation burden by not imposing the complexities of writing settings as well In contrast, implementing ISettingsProvider assumes that there is never a reason to have a class that can write settings without reading them The inheritance relationship between ISettingsProvider and IReadableSettingsProvider, therefore, forces the combined total of both interfaces on the ISettingsProvider class
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Multiple Interface Inheritance
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Just as classes can implement multiple interfaces, interfaces can inherit from multiple interfaces, and the syntax is consistent with class derivation and implementation, as shown in Listing 79
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Listing 79: Multiple Interface Inheritance
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interface IReadableSettingsProvider { string GetSetting(string name, string defaultValue); }
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interface IWriteableSettingsProvider { void SetSetting(string name, string value); }
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interface ISettingsProvider : IReadableSettingsProvider, IWriteableSettingsProvider { }
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Extension Methods on Interfaces
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It is unusual to have an interface with no members, but if implementing both interfaces together is predominant, it is a reasonable choice for this case The difference between Listing 79 and Listing 76 is that it is now possible to implement IWriteableSettingsProvider without supplying any read capability Listing 76 s FileSettingsProvider is unaffected, but if it used explicit member implementation, specifying which interface a member belongs to changes slightly
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Extension Methods on Interfaces
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Perhaps one of the most important features of extension methods is that they work with interfaces in addition to classes The syntax is identical to that of extension methods for classes The extended type (the first parameter and the parameter prefixed with this) is the interface we extend Listing 710 shows an extension method for IListable() It is declared on Listable
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Listing 710: Interface Extension Methods
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class Program { public static void Main() { Contact[] contacts = new Contact[6]; contacts[0] = new Contact( "Dick", "Traci", "123 Main St, Spokane, WA 99037", "123-123-1234"); // // Classes are cast implicitly to // their supported interfaces contactsList(ContactHeaders); ConsoleWriteLine(); Publication[] publications = new Publication[3] { new Publication("Celebration of Discipline", "Richard Foster", 1978), new Publication("Orthodoxy", "GK Chesterton", 1908), new Publication( "The Hitchhiker's Guide to the Galaxy", "Douglas Adams", 1979)
7: Interfaces
}; publicationsList(PublicationHeaders); } }
static class Listable { public static void List( this IListable[] items, string[] headers) { int[] columnWidths = DisplayHeaders(headers); for (int itemCount = 0; itemCount < itemsLength; itemCount++) { string[] values = items[itemCount]ColumnValues; DisplayItemRow(columnWidths, values); } } // count < valuesLength; count++) { ConsoleWrite( "{0}{1,-" + columnWidths[count] + "}", tab, values[count]); { tab = "\t"; } } ConsoleWriteLine(); }
Notice that in this example, the extension method is not for an IListable parameter (although it could have been), but rather an IListable[] parameter This demonstrates that C# allows extension methods not only on an instance of a particular object, but also on a collection of those objects Support for extension methods is the foundation on which LINQ is implemented IEnumerable is the fundamental interface which all collections implement By defining extension methods for IEnumerable, LINQ support was added to all collections This radically changed programming with collections of objects, a topic explored in detail in 14