119: Implementing a Generic Interface in C#.NET

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Listing 119: Implementing a Generic Interface
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public struct Pair<T>: IPair<T> { public T First { get { return _First; } set { _First = value;
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Introducing Generic Types
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} } private T _First; public T Second { get { return _Second; } set { _Second = value; } } private T _Second; }
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Support for generic interfaces is especially important for collection classes, where generics are most prevalent Without generics, developers relied on a series of interfaces within the SystemCollections namespace Like their implementing classes, these interfaces worked only with type object, and as a result, the interface forced all access to and from these collection classes to require a cast By using generic interfaces, you can avoid cast operations, because a stronger compile-time binding can be achieved with parameterized interfaces
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ADVANCED TOPIC Implementing the Same Interface Multiple Times on a Single Class One side effect of template interfaces is that you can implement the same interface many times using different type parameters Consider the IContainer<T> example in Listing 1110
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Listing 1110: Duplicating an Interface Implementation on a Single Class
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public interface IContainer<T> { ICollection<T> Items { get; set; }
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11: Generics
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public class Person: IContainer<Address>, IContainer<Phone>, IContainer<Email> { ICollection<Address> IContainer<Address>Items { get{} set{} } ICollection<Phone> IContainer<Phone>Items { get{} set{} } ICollection<Email> IContainer<Email>Items { get{} set{} } }
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In this example, the Items property appears multiple times using an explicit interface implementation with a varying type parameter Without generics, this is not possible, and instead, the compiler would allow only one explicit IContainerItems property One possible improvement on Person would be to also implement IContainer<object> and to have items return the combination of all three containers (Address, Phone, and Email)
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Defining a Constructor and a Finalizer Perhaps surprisingly, the constructor and destructor on a generic do not require type parameters in order to match the class declaration (ie, not Pair<T>(){}) In the pair example in Listing 1111, the constructor is declared using public Pair(T first, T second)
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Listing 1111: Declaring a Generic Type s Constructor
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public struct Pair<T>: IPair<T> { public Pair(T first, T second) { _First = first; _Second = second; }
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public T First { get{ return _First; } set{ _First = value; } } private T _First; public T Second { get{ return _Second; } set{ _Second = value; } } private T _Second; }
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Specifying a Default Value Listing 111 included a constructor that takes the initial values for both First and Second, and assigns them to _First and _Second Since Pair<T> is a struct, any constructor you provide must initialize all fields This presents a problem, however Consider a constructor for Pair<T> that initializes only half of the pair at instantiation time Defining such a constructor, as shown in Listing 1112, causes a compile error because the field _Second goes uninitialized at the end of the constructor Providing initialization for _Second presents a problem since you don t know the data type of T If it is a reference type, then null would work, but this would not suffice if T were a value type (unless it was nullable)
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Listing 1112: Not Initializing All Fields, Causing a Compile Error
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public struct Pair<T>: IPair<T> { // ERROR: Field 'Pair<T>_second' must be fully assigned // before control leaves the constructor // public Pair(T first) // { // _First = first; // } // }
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To deal with this scenario, C# 20 allows a dynamic way to code the default value of any data type using the default operator, first discussed in
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11: Generics
8 In 8, I showed how the default value of int could be specified with default(int) while the default value of a string uses default(string) (which returns null, as it would for all reference types) In the case of T, which _Second requires, you use default(T) (see Listing 1113)
Listing 1113: Initializing a Field with the default Operator
public struct Pair<T>: IPair<T> { public Pair(T first) { _First = first; _Second = default(T); } // }