The results of Listing 77 appear in Output 72 in C#

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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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7: Interfaces
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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) which is derived from a base interface (IReadableSettingsProvider), the class can still declare an 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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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
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Multiple Interface Inheritance IReadableSettingsProvider, therefore, forces the combined total of both
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interfaces on the ISettingsProvider class One final but important note: Although inheritance is the correct term, conceptually it is more accurate to realize that an interface represents a contract; and one contract is allowed to specify that the provisions of another contract must also be followed So, the code ISettingsProvider : IReadableSettingsProvider conceptually states that the ISettingsProvider contract requires also respecting the IReadableSettingsProvider contract rather than that the ISettingsProvider is a kind of IReadableSettingsProvider That being said, the remainder of the chapter will continue using the inheritance relationship terminology in accordance with the standard C# terminology
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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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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
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7: Interfaces
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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
Perhaps one of the most important features of extension methods is the fact 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 that we extend Listing 710 shows an extension method for IListable() It is declared on Listable
Listing 710: Interface Extension Methods
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 implicitly converted 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) }; publicationsList(PublicationHeaders); } }