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C# s and the CLR s support of generic interfaces offers many great features for developers . In this section, I d like to discuss the benefits offered when using generic interfaces . First, generic interfaces offer great compile-time type safety . Some interfaces (such as the non-generic IComparable interface) define methods that have Object parameters or return
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Part II Designing Types
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types . When code calls these interface methods, a reference to an instance of any type can be passed . But this is usually not desired . The following code demonstrates:
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private void SomeMethod1() { Int32 x = 1, y = 2; IComparable c = x; // CompareTo expects an Object; passing y (an Int32) is OK c.CompareTo(y); // y is boxed here // CompareTo expects an Object; passing "2" (a String) compiles // but an ArgumentException is thrown at runtime c.CompareTo("2"); }
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Obviously, it is preferable to have the interface method strongly typed, and this is why the FCL includes a generic IComparable<in T> interface . Here is the new version of the code revised by using the generic interface:
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private void SomeMethod2() { Int32 x = 1, y = 2; IComparable<Int32> c = x; // CompareTo expects an Int32; passing y (an Int32) is OK c.CompareTo(y); // y is not boxed here // CompareTo expects an Int32; passing "2" (a String) results // in a compiler error indicating that String cannot be cast to an Int32 c.CompareTo("2"); // Error }
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The second benefit of generic interfaces is that much less boxing will occur when working with value types . Notice in SomeMethod1 that the non-generic IComparable interface s CompareTo method expects an Object; passing y (an Int32 value type) causes the value in y to be boxed . However, in SomeMethod2, the generic IComparable<in T> interface s CompareTo method expects an Int32; passing y causes it to be passed by value, and no boxing is necessary . Note The FCL defines non-generic and generic versions of the IComparable, ICollection,
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IList, and IDictionary interfaces, as well as some others . If you are defining a type, and
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you want to implement any of these interfaces, you should typically implement the generic versions of these interfaces . The non-generic versions are in the FCL for backward compatibility to work with code written before the .NET Framework supported generics . The non-generic versions also provide users a way of manipulating the data in a more general, less type-safe fashion . Some of the generic interfaces inherit the non-generic versions, so your class will have to implement both the generic and non-generic versions of the interfaces . For example, the generic IEnumerable<out T> interface inherits the non-generic IEnumerable interface . So if your class implements IEnumerable<out T>, your class must also implement IEnumerable .
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13 Interfaces
Sometimes when integrating with other code, you may have to implement a non-generic interface because a generic version of the interface simply doesn t exist . In this case, if any of the interface s methods take or return Object, you will lose compile-time type safety, and you will get boxing with value types . You can alleviate this situation to some extent by using a technique I describe in the Improving Compile-Time Type Safety with Explicit Interface Method Implementations section near the end of this chapter .
The third benefit of generic interfaces is that a class can implement the same interface multiple times as long as different type parameters are used . The following code shows an example of how useful this could be:
using System; // This class implements the generic IComparable<T> interface twice public sealed class Number: IComparable<Int32>, IComparable<String> { private Int32 m_val = 5; // This method implements IComparable<Int32>'s CompareTo public Int32 CompareTo(Int32 n) { return m_val.CompareTo(n); } // This method implements IComparable<String>'s CompareTo public Int32 CompareTo(String s) { return m_val.CompareTo(Int32.Parse(s)); } } public static class Program { public static void Main() { Number n = new Number(); // Here, I compare the value in n with an Int32 (5) IComparable<Int32> cInt32 = n; Int32 result = cInt32.CompareTo(5); // Here, I compare the value in n with a String ("5") IComparable<String> cString = n; result = cString.CompareTo("5"); } }
An interface s generic type parameters can also be marked as contravariant and covariant, which allows even more flexibility for using generic interfaces . For more about contravariance and covariance, see the Delegate and Interface Contravariant and Covariant Generic Type Arguments section in 12 .
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