1119: Needing the Type Parameter to Support an Interface in Visual C#.NET

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Listing 1119: Needing the Type Parameter to Support an Interface
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public class BinaryTree<T> { public Pair<BinaryTree<T>> SubItems { get{ return _SubItems; } set
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{ IComparable first; // ERROR: Cannot implicitly convert type first = valueFirstItem; // Explicit cast required if (firstCompareTo(valueSecondItem) < 0) { // first is less than second } else { // first and second are the same or // second is less than first } _SubItems = value; } } private Pair<BinaryTree<T>> _SubItems; }
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At compile time, the type parameter T is generic When the code is written as shown, the compiler assumes that the only members available on T are those inherited from the base type object, since every type has object as an ancestor (Only methods such as ToString(), therefore, are available to the key instance of the type parameter T) As a result, the compiler displays a compilation error because the CompareTo() method is not defined on type object You can cast the T parameter to the IComparable interface in order to access the CompareTo() method, as shown in Listing 1120
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Listing 1120: Needing the Type Parameter to Support an Interface or Exception Thrown
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public class BinaryTree<T> { public Pair<BinaryTree<T>> SubItems { get{ return _SubItems; } set { IComparable first; first = (IComparable)valueFirstItem; if (firstCompareTo(valueSecondItem) < 0) { // first is less than second
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Constraints
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} else { // second is less than or equal to first } _SubItems = value; } } private Pair<BinaryTree<T>> _SubItems; }
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Unfortunately, however, if you now declare a BinaryTree class variable and supply a type parameter that does not implement the IComparable interface, you encounter an execution-time error specifically, an InvalidCastException This defeats an advantage of generics To avoid this exception and instead provide a compile-time error, C# enables you to supply an optional list of constraints for each type parameter declared in the generic class A constraint declares the type parameter characteristics the generic requires You declare a constraint using the where keyword, followed by a parameter-requirements pair, where the parameter must be one of those defined in the generic type and the requirements are to restrict the class or interface from which the type derives, the presence of a default constructor, or a reference/value type restriction
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Interface Constraints In order to satisfy the sort requirement, you need to use the CompareTo() method in the BinaryTree class To do this most effectively, you impose a constraint on the T type parameter You need the T type parameter to implement the IComparable interface The syntax for this appears in Listing 1121
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Listing 1121: Declaring an Interface Constraint
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public class BinaryTree<T> where T: SystemIComparable { public Pair<BinaryTree<T>> SubItems {
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get{ return _SubItems; } set { IComparable first; // Notice that the cast can now be eliminated first = valueFirstItem; if (firstCompareTo(valueSecondItem) < 0) { // first is less than second } else { // second is less than or equal to first } _SubItems = value; } } private Pair<BinaryTree<T>> _SubItems; }
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Given the interface constraint addition in Listing 1121, the compiler ensures that each time you use the BinaryTree class you specify a type parameter that implements the IComparable interface Furthermore, you no longer need to explicitly cast the variable to an IComparable interface before calling the CompareTo() method Casting is not even required to access members that use explicit interface implementation, which in other contexts would hide the member without a cast To resolve what member to call, the compiler first checks class members directly, and then looks at the explicit interface members If no constraint resolves the argument, only members of object are allowable If you tried to create a BinaryTree<T> variable using SystemText StringBuilder as the type parameter, you would receive a compiler error because StringBuilder does not implement IComparable The error is similar to the one shown in Output 113
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OUTPUT 113:
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error CS0309: The type SystemTextStringBuilder must be convertible to SystemIComparable in order to use it as parameter T in the generic type or method BinaryTree<T>
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