1133: Using a Factory Interface in Place of a Constructor Constraint in Visual C#

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Listing 1133: Using a Factory Interface in Place of a Constructor Constraint
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public class EntityBase<TKey> { public EntityBase(TKey key) { Key = key; } public TKey Key { get { return _key; } set { _key = value; } } private TKey _key; } public class EntityDictionary<TKey, TValue, TFactory> : Dictionary<TKey, TValue> where TKey : IComparable<T>, IFormattable
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where TValue : EntityBase<TKey> where TFactory : IEntityFactory<TKey, TValue>, new() { public TValue New(TKey key) { TValue newEntity = new TFactory()CreateNew(key); Add(newEntityKey, newEntity); return newEntity; } } public interface IEntityFactory<TKey, TValue> { TValue CreateNew(TKey key); }
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A declaration such as this allows you to pass the new key to a TValue constructor that takes parameters rather than the default constructor It no longer uses the constructor constraint on TValue because TFactory is responsible for instantiating the order instead of EntityDictionary<> (One modification to the code in Listing 1133 would be to save a copy of the factory This would enable you to reuse the factory instead of reinstantiating it every time) A declaration for a variable of type EntityDictionary<TKey, TValue, TFactory> would result in an entity declaration similar to the Order entity in Listing 1134
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Listing 1134: Declaring an Entity to Be Used in EntityDictionary<>
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public class Order : EntityBase<Guid> { public Order(Guid key) : base(key) { // } } public class OrderFactory : IEntityFactory<Guid, Order> { public Order CreateNew(Guid key) { return new Order(key); } }
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Generic Methods
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Generic Methods
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You already learned that it is relatively simple to add a generic method to a class when the class is a generic You did this in the generic class examples so far, and it also works for static methods Furthermore, you can use generic classes within a generic class, as you did in earlier BinaryTree listings using the following line of code:
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public Pair< BinaryTree<T> > SubItems;
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Generic methods are methods that use generics even when the containing class is not a generic class or the method contains type parameters not included in the generic class type parameter list To define generic methods, you add the type parameter syntax immediately following the method name, as shown in the MathExMax<T> and MathExMin<T> examples in Listing 1135
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Listing 1135: Defining Generic Methods
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public static class MathEx { public static T Max<T>(T first, params T[] values) where T : IComparable<T> { T maximum = first; foreach (T item in values) { if (itemCompareTo(maximum) > 0) { maximum = item; } } return maximum; } public static T Min<T>(T first, params T[] values) where T : IComparable<T> { T minimum = first; foreach (T item in values) { if (itemCompareTo(minimum) < 0) { minimum = item; } }
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return minimum; } }
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You use the same syntax on a generic class when the method requires an additional type parameter not included in the class type parameter list In this example, the method is static but C# does not require this Note that generic methods, like classes, can include more than one type parameter The arity (the number of type parameters) is an additional distinguishing characteristic of a method signature
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Type Inferencing The code used to call the Min<T> and Max<T> methods looks like that shown in Listing 1136
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Listing 1136: Specifying the Type Parameter Explicitly
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ConsoleWriteLine( MathExMax<int>(7, 490)); ConsoleWriteLine( MathExMin<string>("ROUS", "Fireswamp"));
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The output to Listing 1136 appears in Output 114
OUTPUT 114:
490 Fireswamp
Not surprisingly, the type parameters, int and string, correspond to the actual types used in the generic method calls However, specifying the type is redundant because the compiler can infer the type from the parameters passed to the method To avoid redundancy, you can exclude the type parameters from the call This is known as type inferencing, and an example appears in Listing 1137 The output appears in Output 115
Listing 1137: Inferring the Type Parameter
ConsoleWriteLine( MathExMax(7, 490)); ConsoleWriteLine( MathExMin("ROUS'", "Fireswamp"));
Generic Methods
OUTPUT 115:
490 Fireswamp
For type inferencing to be successful, the types must match the method signature However, starting with C# 30, the compiler added an enhancement to imply the type parameter as long as the types were implicitly compatible For example, calling the Max<T> method using MathExMax(70, 490) will compile successfully Even though the parameters are not both the same type (int and double), they will both implicitly convert to double, so the method call compiles You can resolve the error by either casting explicitly or including the type argument Also note that you cannot perform type inferencing purely on the return type Parameters are required for type inferencing to be allowed
Specifying Constraints The generic method also allows constraints to be specified For example, you can restrict a type parameter to implement IComparable<T> The constraint is specified immediately following the method header, prior to the curly braces of the method block, as shown in Listing 1138