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BEGINNER TOPIC Why Interfaces Implemented interfaces are like appliances with wall plugs The wall plug is the interface that appliances support in order to receive AC power An appliance can use that power in countless ways, but in order to plug into a wall socket, an appliance must supply a compatible wall plug What the appliance does with the power corresponds to how an interface implementation varies from class to class The specification that defines a wall plug is the contract that must be supported for an appliance to plug into the wall plug Similarly, an interface defines a contract that a class must support to gain the capability that the interface provides Consider the following example: An innumerable number of file compression formats are available (zip, 7-zip, cab, lha, tar, targz, tarbz2, bh, rar, arj, arc, ace, zoo, gz, bzip2, xxe, mime, uue, and yenc, just to name a few) If you created classes for each compression format, you could end up with different method signatures for each compression implementation and no ability for a standard calling convention across them Although the method signature could be defined in an abstract member of a base class, deriving from a common base type uses up a class s one and only inheritance, with an unlikely chance of sharing code across the various compression implementations, thereby making the potential of a base class implementation useless Instead of sharing a common base class, each compression class needs to implement a common interface Interfaces define the contract a class supports to interact with the other classes that expect the interface Although there are many potential compression algorithms, if all of them could implement the IFileCompression interface and its Compress() and Uncompress() methods, the code for calling the algorithm on any particular compression class would simply involve a cast to the IFileCompression interface and a call into the members, regardless of which class implemented the methods The result is polymorphism because each compression class has the same method signature but individual implementations of that signature
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The naming convention for interfaces is to use Pascal case, with an I prefix The IFileCompression interface shown in Listing 71 is an example of such a name and interface definition
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Listing 71: Defining an Interface
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interface IFileCompression { void Compress(string targetFileName, string[] fileList); void Uncompress( string compressedFileName, string expandDirectoryName); }
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IFileCompression defines the methods a class implements to work with
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other compression-related classes The power of defining the interface concerns the ability to switch among implementations without modifying the calling code, as long as each compression class implements the IFileCompression interface One key characteristic of an interface is that it has no implementation and no data Method declarations have a single semicolon in place of curly braces after the header Fields (data) cannot appear on an interface When an interface requires the derived class to have certain data, it uses a property rather than a field Since the property does not contain any implementation as part of the interface declaration, it doesn t reference a backing field Given that the purpose of the interface is to define the contract among multiple classes, defining private or protected members would make them inaccessible to other classes, defeating the purpose of the interface Therefore, C# does not allow access modifiers on interface members, and instead it automatically defines them as public
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Consider another example (see Listing 72): IListable defines the members a class needs to support for the ConsoleListControl class to display it As such, any class that implements IListable will have the capability of using the ConsoleListControl to display itself The IListable interface requires a read-only property, ColumnValues
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Listing 72: Implementing and Using Interfaces
interface IListable { // Return the value of each column // in the row string[] ColumnValues { get; } }
public abstract class PdaItem { public PdaItem(string name) { Name = name; } public virtual string Name{get;set;} }
class Contact : PdaItem, IListable { public Contact(string firstName, string lastName, string address, string phone) : base(null) { FirstName = firstName; LastName = lastName; Address = address; Phone = phone; } public public public public string string string string FirstName { get; set; } LastName { get; set; } Address { get; set; } Phone { get; set; }
public string[] ColumnValues { get { return new string[] { FirstName, LastName, Phone, Address }; } }