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Multiple Constraints For any given type parameter, you may specify any number of interfaces as constraints, but no more than one class, just as a class may implement any number of interfaces but inherit from only one other class Each new constraint is declared in a comma-delimited list following the generic type and a colon If there is more than one type parameter, each must be preceded by the where keyword In Listing 1125, the EntityDictionary class contains two type parameters: TKey and TValue The TKey type parameter has two interface constraints, and the TValue type parameter has one base class constraint
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Listing 1125: Specifying Multiple Constraints
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public class EntityDictionary<TKey, TValue> : Dictionary<TKey, TValue> where TKey : IComparable<TKey>, IFormattable where TValue : EntityBase { }
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In this case, there are multiple constraints on TKey itself and an additional constraint on TValue When specifying multiple constraints on one type parameter, an AND relationship is assumed TKey must implement IComparable<TKey> and IFormattable, for example Notice there is no comma between each where clause
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Constructor Constraints In some cases, it is desirable to create an instance of a type parameter inside the generic class In Listing 1126, the New() method for the EntityDictionary<TKey, TValue> class must create an instance of the type parameter TValue
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Listing 1126: Requiring a Default Constructor Constraint
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public class EntityBase<TKey> { public TKey Key { get{ return _Key; } set{ _Key = value; } }
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Constraints
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private TKey _Key; } public class EntityDictionary<TKey, TValue> : Dictionary<TKey, TValue> where TKey: IComparable<TKey>, IFormattable where TValue : EntityBase<TKey>, new() { // public TValue New(TKey key) { TValue newEntity = new TValue(); newEntityKey = key; Add(newEntityKey, newEntity); return newEntity; } // }
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Because not all objects are guaranteed to have public default constructors, the compiler does not allow you to call the default constructor on the type parameter To override this compiler restriction, you add the text new() after all other constraints are specified This text is a constructor constraint, and it forces the type parameter decorated with the constructor constraint to have a default constructor Only the default constructor constraint is available You cannot specify a constraint for a constructor with parameters
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Constraint Inheritance Constraints are inherited by a derived class, but they must be specified explicitly on the derived class Consider Listing 1127
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Listing 1127: Inherited Constraints Specified Explicitly
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class EntityBase<T> where T : IComparable<T> { // }
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// // // //
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ERROR: The type 'T' must be convertible to 'SystemIComparable<T>' in order to use it as parameter 'T' in the generic type or method
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// class Entity<T> : EntityBase<T> // { // // }
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Because EntityBase requires that T implement IComparable<T>, the Entity class needs to explicitly include the same constraint Failure to do so will result in a compile error This increases a programmer s awareness of the constraint in the derived class, avoiding confusion when using the derived class and discovering the constraint but not understanding where it comes from In contrast, constraints on generic override (or explicit interface) methods are inherited implicitly and may not be restated (see Listing 1128)
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Listing 1128: Inherited Constraints Specified Explicitly
class EntityBase<T> where T : IComparable<T> { public virtual void Method<T>(T t) where T : IComparable<T> { // } }
class Entity<T> : EntityBase<T> { public virtual void Method<T>(T t) // Error: Constraints may not be // repeated on overriding members where T : IComparable<T> { // } }
In the inheritance case the type parameter on the base class can be additionally constrained by adding not only the constraints on the base class (required), but also additional constraints as well However, overriding members need to conform to the interface defined in the base class method Additional constraints could break polymorphism, so they are not allowed and the type parameter constraints on the override method are implied