Working around Single Inheritance Using Aggregation with Interfaces in Visual C#.NET

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Listing 710 Working around Single Inheritance Using Aggregation with Interfaces
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public class PdaItem { // protected Guid ObjectKey { get { return _ObjectKey; } set { _ObjectKey = value; } } private Guid _ObjectKey; public virtual string Name { get { return _Name; } set { _Name = value; } } private string _Name; public string DateTimeLastUpdate { get { return _DateTimeLastUpdate; } set { _DateTimeLastUpdate = value; } } private string _DateTimeLastUpdate; } interface IPerson { string FirstName { get; set; } string LastName { get; set; } }
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public class Person : IPerson { // public string FirstName { get { return _FirstName; } set { _FirstName = value; } } private string _FirstName; public string LastName { get { return _LastName; } set { _LastName = value; } } private string _LastName; } public class Contact : PdaItem, IPerson { private Person Person { get { return _Person; } set { _Person = value; } } private Person _Person;
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public string FirstName { get { return _PersonFirstName; } set { _PersonFirstName = value } } public string LastName { get { return _PersonLastName; } set { _PersonLastName = value; } } // public string Address { get { return _Address; } set { _Address = value; } } private string _Address; public string Phone { get { return _Phone; } set { _Phone = value; } } private string _Phone; }
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IPerson ensures that the signatures between the Person members and the same members duplicated
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onto Contact are consistent The implementation is still not synonymous with multiple inheritance, however, because new members added to Person will not be added to Contact
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Beginner Topic: Interface Diagramming
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Interfaces in a UML-like figure take two possible forms First, you can show the interface as though it is an inheritance relationship similar to a class inheritance, as demonstrated in Figure 71 between IPerson and IContact Alternatively, you can show the interface using a small circle, often referred to as a lollipop, exemplified by IPerson and IContact in Figure 71
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Figure 71 Working around Single Inheritances with Aggregation and Interfaces
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In Figure 71, Contact derives from PdaItem and implements IContact In addition, it aggregates the Person class, which implements IPerson Although the Visual Studio 2005 Class Designer does not support this, interfaces are sometimes shown as using a derivation-type arrow to a class For example, Person could have an arrow to IPerson instead of a lollipop
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Versioning
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When creating a new version of a component or application that other developers have programmed against, you should not change interfaces Because interfaces define a contract between the implementing class and the class using the interface, changing the interface is changing the contract, which will possibly break any code written against the interface Changing or removing a particular interface member signature is obviously a code-breaking change, as any call to that member will no longer compile without modification The same is true when changing public or protected member signatures on a class However, unlike with classes, adding members to an interface could also prevent code from compiling without additional changes The problem is that any class implementing the interface must do so entirely, and implementations for all members must be provided With new interface members, the compiler will require that developers add new interface members to the class implementing the interface The creation of IDistributedSettingsProvider in Listing 711 serves as a good example of extending an interface in a version-compatible way Imagine that at first, only the ISettingsProvider interface is defined (as it was in Listing 76) In the next version, however, it is determined that per-machine settings are required To enable this, the IDistributedSettingsProvider interface is created, and it derives from ISettingsProvider
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Listing 711 Deriving One Interface from Another
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interface IDistributedSettingsProvider : ISettingsProvider { /// <summary> /// Get the settings for a particular machine /// </summary> /// <param name="machineName"> /// The machine name the setting is related to</param> /// <param name="name">The name of the setting</param> /// <param name="defaultValue"> /// The value returned if the setting is not found</param> /// <returns>The specified setting</returns> string GetSetting( string machineName, string name, string defaultValue); /// /// /// /// /// /// /// /// <summary> Set the settings for a particular machine </summary> <param name="machineName"> The machine name the setting is related to</param> <param name="name">The name of the setting</param> <param name="value">The value to be persisted</param> <returns>The specified setting</returns>
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void SetSetting( string machineName, string name, string value); }
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The important factor is that programmers with classes that implement ISettingsProvider can choose to upgrade the implementation to include IDistributedSettingsProvider , or they can ignore it If instead of creating a new interface, the machine-related methods are added to ISettingsProvider, then classes implementing this interface will no longer successfully compile with the new interface definition, and instead a version-breaking change will occur Changing interfaces during the development phase is obviously acceptable, although perhaps laborious if implemented extensively However, once an interface is released, it should not be changed Instead, a second interface should be created, possibly deriving from the original interface (Listing 711 includes XML comments describing the interface members, as discussed further in 9)
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