Constraints in C#

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implement the IComparable<T> interface The syntax for this appears in Listing 1122
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Listing 1122: Declaring an Interface Constraint
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public class BinaryTree<T> where T: SystemIComparable<T> { public Pair<BinaryTree<T>> SubItems { get{ return _SubItems; } set { IComparable<T> 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 1122, the compiler ensures that each time you use the BinaryTree class you specify a type parameter that implements the IComparable<T> interface Furthermore, you no longer need to explicitly cast the variable to an IComparable<T> 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
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11: Generics
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If you tried to create a BinaryTree<T> variable using System TextStringBuilder as the type parameter, you would receive a compiler error because StringBuilder does not implement IComparable<T> 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<T> in order to use it as parameter T in the generic type or method BinaryTree<T>
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To specify an interface for the constraint you declare an interface constraint This constraint even circumvents the need to cast in order to call an explicit interface member implementation
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Base Class Constraints Sometimes you might want to limit the constructed type to a particular class derivation You do this using a base class constraint, as shown in Listing 1123
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Listing 1123: Declaring a Base Class Constraint
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public class EntityDictionary<TKey, TValue> : SystemCollectionsGenericDictionary<TKey, TValue> where TValue : EntityBase { }
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In contrast to SystemCollectionsGenericDictionary<TKey, TValue> on its own, EntityDictionary<TKey, TValue> requires that all TValue types derive from the EntityBase class By requiring the derivation, it is possible to always perform a cast operation within the generic implementation, because the constraint will ensure that all type parameters derive from the base and, therefore, that all TValue type parameters used with EntityDictionary can be implicitly converted to the base The syntax for the base class constraint is the same as that for the interface constraint, except that base class constraints must appear first when multiple constraints are specified However, unlike interface constraints,
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Constraints
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multiple base class constraints are not allowed since it is not possible to derive from multiple classes Similarly, base class constraints cannot be specified for sealed classes or specific structs For example, C# does not allow a constraint for a type parameter to be derived from string or SystemNullable<T>
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struct/class Constraints Another valuable generic constraint is the ability to restrict type parameters to a value type or a reference type The compiler does not allow specifying SystemValueType as the base class in a constraint Instead, C# provides special syntax that works for reference types as well Instead of specifying a class from which T must derive, you simply use the keyword struct or class, as shown in Listing 1124
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Listing 1124: Specifying the Type Parameter As a Value Type
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public struct Nullable<T> : IFormattable, IComparable, IComparable<Nullable<T>>, INullable where T : struct { // }
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Because a base class constraint requires a particular base class, using struct or class with a base class constraint would be pointless, and in fact could allow for conflicting constraints Therefore, you cannot use struct and class constraints with a base class constraint There is one special characteristic for the struct constraint It limits possible type parameters as being only value types while at the same time preventing type parameters that are SystemNullable<T> type parameters Why Without this last restriction, it would be possible to define the nonsense type Nullable<Nullable<T>>, which is nonsense because Nullable<T> on its own allows a value type variable that supports nulls, so a nullable-nullable type becomes meaningless Since the nullable operator ( ) is a C# shortcut for declaring a nullable value type, the Nullable<T> restriction provided by the struct constraint also prevents code such as the following:
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