Created
August 2, 2015 07:16
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using System; | |
using System.Linq; | |
using System.Reflection; | |
namespace NullObjectDesignPattern | |
{ | |
class Program | |
{ | |
static void Main(string[] args) | |
{ | |
// parameterless -- falls back to default dog | |
var dogFactory = new Factory<Dog>(new Dog { Name = "Tommy" }); | |
var dog1 = dogFactory.Create<Dog>("Bruno"); | |
Console.WriteLine(dog1.Name); | |
// parameterless -- overrides default dog | |
var dog2 = dogFactory.Create<Dog>(); | |
Console.WriteLine(dog2.Name); | |
// parameterless -- falls back to default Cat | |
var catFactory = new Factory<Cat>(new Cat("Lisa")); | |
var cat1 = catFactory.Create<Cat>(); | |
Console.WriteLine(cat1.Name); | |
// parameterized -- overrides default Cat | |
var cat2 = catFactory.Create<Cat>("Polly"); | |
Console.WriteLine(cat2.Name); | |
// parameterized -- falls back to default Dog but creates a more derived type | |
var dog3 = dogFactory.Create<Poodle>(); | |
Console.WriteLine($"Name: {dog3.Name}, Type {dog3.GetType().Name}"); | |
// parameterized -- overrides default Dog | |
var dog4 = dogFactory.Create<Poodle>(); | |
Console.WriteLine($"Name: {dog4.Name}, Type {dog4.GetType().Name}"); | |
Console.ReadKey(); | |
} | |
} | |
class Factory<T> where T: class | |
{ | |
private T _defaultValue = default(T); | |
public Factory(T defaultValue = default(T)) | |
{ | |
_defaultValue = defaultValue; | |
} | |
public T Create<Derived>(params object[] args) where Derived : T | |
{ | |
return TryCreate<Derived>(args) ?? _defaultValue; | |
} | |
protected virtual T TryCreate<Derived>(params object[] args) where Derived : T | |
{ | |
var matchingCtor = (from ctor in typeof(Derived).GetConstructors() | |
let parameters = ctor.GetParameters() | |
where parameters.Length == args.Length | |
where parameters.AndArgumentTypesAreSame(args) | |
select ctor).FirstOrDefault(); | |
if (matchingCtor == null) return null; | |
return matchingCtor.Invoke(args) as T; | |
} | |
} | |
public static class Extensions | |
{ | |
public static bool AndArgumentTypesAreSame(this ParameterInfo[] parameterInfoArray, object[] argumentArray) | |
{ | |
if (parameterInfoArray != null && argumentArray != null && parameterInfoArray.Length == argumentArray.Length) | |
{ | |
for(int i = 0; i < parameterInfoArray.Length; i++) | |
{ | |
if (parameterInfoArray[i].ParameterType != argumentArray[i].GetType()) | |
return false; | |
} | |
return true; | |
} | |
return false; | |
} | |
} | |
class Dog | |
{ | |
public string Name { get; set; } = "Woofy"; | |
} | |
class Poodle : Dog { } | |
class Cat | |
{ | |
public string Name { get; set; } | |
public Cat(string name) | |
{ | |
Name = name; | |
} | |
} | |
} |
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