Interface Inheritance in Visual C#.NET

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Interfaces can derive from each other, resulting in an interface that inherits all the members in its base interfaces As shown in Listing 76, the interfaces directly derived from IReadableSettingsProvider are the explicit base interfaces
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Listing 76 Deriving One Interface from Another
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interface IReadableSettingsProvider { string GetSetting(string name, string defaultValue); } interface ISettingsProvider : IReadableSettingsProvider { void SetSetting(string name, string value); } class FileSettingsProvider : ISettingsProvider { #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 case, ISettingsProvider derives from IReadableSettingsProvider and, therefore, inherits its
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members If IReadableSettingsProvider also had an explicit base interface, ISettingsProvider would inherit those members too, and the full set of interfaces in the derivation hierarchy would simply be the accumulation of base interfaces It is interesting to note that if GetSetting() is implemented explicitly, it must be done using IReadableSettingsProvider The declaration with ISettingsProvider in Listing 77 will not compile
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Listing 77 Explicit Member Declaration without the Containing Interface (Failure)
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// ERROR: GetSetting() not available on ISettingsProvider string ISettingsProviderGetSetting( string name, string defaultValue) { // }
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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 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 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 only required 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); } interface IWriteableSettingsProvider { void SetSetting(string name, string value); } 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 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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Implementing Multiple Inheritance via Interfaces
As Listing 73 demonstrated, a single class can implement any number of interfaces in addition to deriving from a single class This feature provides a possible enhancement to work around the lack of multiple inheritance support in C# classes The process uses aggregation as described in the previous chapter, but you can vary the structure slightly by adding an interface to the mix, as shown in Listing 710