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Name of object referred to by AlphaRef is Alpha #1 Name of object referred to by AlphaRef is now Beta #1
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It is important to stress that AlphaRef can be assigned a reference to a MyClass<Beta> object only because T is covariant in IMyCoVarGenIF To prove this, remove out from in IMyCoVarGenIF s declaration of T, and then attempt to recompile the program The compilation will fail because the default strict type-checking will not allow the assignment It is possible for one generic interface to be inherited by another In other words, a generic interface with a covariant type parameter can be extended For example,
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public interface IMyCoVarGenIF2<out T> : IMyCoVarGenIF<T> { // }
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Part I:
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Notice that out is specified only in the extending interface s declaration Specifying out in the base interface clause is not necessary or legal One last point: it is legal for IMyCoVarGenIF2 to not specify T as covariant However, doing so eliminates the covariance that extending IMyCoVarGetIF could provide Of course, making IMyCoVarGenIF2 invariant may be required for some uses Here are some restrictions that apply to covariance A covariant type parameter can be applied only to a method return type Thus, out cannot be applied to a type parameter that is used to declare a method parameter Covariance works only with reference types A covariant type cannot be used as a constraint in an interface method For example, this interface is illegal:
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public interface IMyCoVarGenIF2<out T> { void M<V>() where V:T; // Error, covariant T cannot be used as constraint }
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Using Contravariance in a Generic Interface
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As it applies to a generic interface, contravariance is the feature that lets a method use an argument whose type is a base class of the type specified by the type parameter for that parameter In the past, because of the strict type-checking applied to generics, a method s argument type had to match the type parameter precisely Contravariance relaxes this rule in a type-safe way A contravariant type parameter is declared by preceding the type parameter with the keyword in To understand the effects of contravariance, we will again work through an example To begin, here is a contravariant generic interface called IMyContraVarGenIF Notice that its type parameter T is contravariant and it uses T in the declaration of a method called Show( )
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// This generic interface supports contravariance public interface IMyContraVarGenIF<in T> { void Show(T obj); }
Notice that T is specified as contravariant by preceding it with in Also, notice that the parameter type of obj is T Next, MyClass implements IMyContraVarGenIF, as shown here:
// Implement the IMyContraVarGenIF interface class MyClass<T> : IMyContraVarGenIF<T> { public void Show(T x) { ConsoleWriteLine(x); } }
Here, Show( ) simply displays the string representation of x (as obtained by WriteLine( ) s implicit call to ToString( )) Next, a class hierarchy is declared:
// Create a simple class hierarchy class Alpha { public override string ToString() { return "This is an Alpha object"; }
18:
Generics
// }
PART I
class Beta : Alpha { public override string ToString() { return "This is a Beta object"; } // }
Notice that these versions of Alpha and Beta differ from the previous example for the sake of illustration Also notice that ToString( ) is overridden to return the type of object Given the foregoing, the following sequence is legal:
// Create an IMyContraVarGenIF<Alpha> reference to a // MyClass<Alpha> object // This is legal with or without contravariance IMyContraVarGenIF<Alpha> AlphaRef = new MyClass<Alpha>(); // Create an IMyContraVarGenIF<beta> reference to a // MyClass<Beta> object // This is legal with or without contravariance IMyContraVarGenIF<Beta> BetaRef = new MyClass<Beta>(); // Create an IMyContraVarGenIF<beta> reference to // a MyClass<Alpha> object // *** This is legal because of contravariance *** IMyContraVarGenIF<Beta> BetaRef2 = new MyClass<Alpha>(); // This call is legal with or without contravariance BetaRefShow(new Beta()); // Assign AlphaRef to BetaRef // *** This is legal because of contravariance *** BetaRef = AlphaRef; BetaRefShow(new Beta());
First, notice that two IMyContraVarGenIF reference variables are created and are assigned references to MyClass objects whose type parameters match that of the interface references The first uses Alpha The second uses Beta These declarations do not require contravariance and are legal in all cases Next, an IMyContraVarGenIF<Beta> reference is created, but it is assigned a reference to a MyClass<Alpha> object This is legal only because T is contravariant As you would expect, the next line, which calls BetaRefShow( ) with a Beta argument, is legal because T in MyClass<Beta> is Beta, and the argument to Show( ) is Beta The next line assigns AlphaRef to BetaRef This is legal only because of contravariance In this case, BetaRef is of type MyClass<Beta>, but AlphaRef is of type MyClass<Alpha> Because Alpha is a base class of Beta, contravariance makes this conversion legal To prove to yourself that contravariance is required in the program, try removing in from the declaration of T in IMyContraVarGenIF Then attempt to recompile the program As you will see, errors will result
Part I:
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