Generische Singleton<T>

Das problem mit einer generischen singleton-factory ist, dass seit es ist generisch, Sie haben keine Kontrolle über die "singleton" type instanziiert wird, so dass Sie nie garantieren können, dass die Instanz, die Sie erstellen, wird die einzige Instanz in der Anwendung.

Wenn ein user kann ein Typ als eine generische typargument dann können Sie auch erstellen von Instanzen dieses Typs. In anderen Worten, Sie können nicht erstellen Sie eine generische singleton-factory - es untergräbt das Muster selbst.

InformationsquelleAutor Andrew Hare

Hier ist meine Umsetzung mit einem nicht-öffentlichen Konstruktor. Das einzige Problem jetzt ist es gibt keine Möglichkeit, eine benutzerdefinierte Einschränkung auf C# - Generika, also ich werfe eine Laufzeitausnahme für abgeleitete Klassen mit öffentlichen default-Konstruktoren statt einer compile-Zeit-Fehler.

using System;
using System.Reflection;
using System.Threading;

///<summary>
///A generic abstract implementation of the Singleton design pattern (http://en.wikipedia.org/wiki/Singleton_pattern).
///
///Derived type must contain a non-public default constructor to satisfy the rules of the Singleton Pattern.
///If no matching constructor is found, an exception will be thrown at run-time. I am working on a StyleCop
///constraint that will throw a compile-time error in the future.
///
///Example Usage (C#):
///
///    class MySingleton : Singleton&lt;MySingleton&gt;
///    {
///        private const string HelloWorldMessage = "Hello World - from MySingleton";
///    
///        public string HelloWorld { get; private set; }
///
///        //Note: *** Private Constructor ***
///        private MySingleton()
///        {
///            //Set default message here.
///            HelloWorld = HelloWorldMessage;
///        }
///    }
///
///    class Program
///    {
///        static void Main()
///        {
///            var mySingleton = MySingleton.Instance;
///            Console.WriteLine(mySingleton.HelloWorld);
///            Console.ReadKey();
///        }
///    }
///</summary>
///<typeparam name="T">Type of derived Singleton object (i.e. class MySingletone: Singleton&lt;MySingleton&gt;).</typeparam>
public abstract class Singleton<T> where T : class
{
    ///<summary>
    ///"_instance" is the meat of the Singleton<T> base-class, as it both holds the instance
    ///pointer and the reflection based factory class used by Lazy&lt;T&gt; for instantiation.
    ///
    ///Lazy&lt;T&gt;.ctor(Func&lt;T&gt; valueFactory,LazyThreadSafetyMode mode), valueFactory:
    ///
    ///    Due to the fact Lazy&lt;T&gt; cannot access a singleton's (non-public) default constructor and
    ///    there is no "non-public default constructor required" constraint available for C# 
    ///    generic types, Lazy&lt;T&gt;'s valueFactory Lambda uses reflection to create the instance.
    ///
    ///Lazy&lt;T&gt;.ctor(Func&lt;T&gt; valueFactory,LazyThreadSafetyMode mode), mode:
    ///
    ///    Explanation of selected mode (ExecutionAndPublication) is from MSDN.
    ///    
    ///    Locks are used to ensure that only a single thread can initialize a Lazy&lt;T&gt; instance 
    ///    in a thread-safe manner. If the initialization method (or the default constructor, if 
    ///    there is no initialization method) uses locks internally, deadlocks can occur. If you 
    ///    use a Lazy&lt;T&gt; constructor that specifies an initialization method (valueFactory parameter),
    ///    and if that initialization method throws an exception (or fails to handle an exception) the 
    ///    first time you call the Lazy&lt;T&gt;.Value property, then the exception is cached and thrown
    ///    again on subsequent calls to the Lazy&lt;T&gt;.Value property. If you use a Lazy&lt;T&gt; 
    ///    constructor that does not specify an initialization method, exceptions that are thrown by
    ///    the default constructor for T are not cached. In that case, a subsequent call to the 
    ///    Lazy&lt;T&gt;.Value property might successfully initialize the Lazy&lt;T&gt; instance. If the
    ///    initialization method recursively accesses the Value property of the Lazy&lt;T&gt; instance,
    ///    an InvalidOperationException is thrown.
    ///
    ///</summary>
    private static readonly Lazy<T> _instance = new Lazy<T>(() =>
                                                                {
                                                                    //Get non-public constructors for T.
                                                                    var ctors = typeof (T).GetConstructors(BindingFlags.Instance | BindingFlags.NonPublic);

                                                                    //If we can't find the right type of construcor, throw an exception.
                                                                    if (!Array.Exists(ctors, (ci) => ci.GetParameters().Length == 0))
                                                                    {
                                                                        throw new ConstructorNotFoundException("Non-public ctor() note found.");
                                                                    }

                                                                    //Get reference to default non-public constructor.
                                                                    var ctor = Array.Find(ctors, (ci) => ci.GetParameters().Length == 0);

                                                                    //Invoke constructor and return resulting object.
                                                                    return ctor.Invoke(new object[] {}) as T;
                                                                }, LazyThreadSafetyMode.ExecutionAndPublication);

    ///<summary>
    ///Singleton instance access property.
    ///</summary>
    public static T Instance
    {
        get { return _instance.Value; }
    }
}

///<summary>
///Exception thrown by Singleton&lt;T&gt; when derived type does not contain a non-public default constructor.
///</summary>
public class ConstructorNotFoundException : Exception
{
    private const string ConstructorNotFoundMessage = "Singleton<T> derived types require a non-public default constructor.";
    public ConstructorNotFoundException() : base(ConstructorNotFoundMessage) { }
    public ConstructorNotFoundException(string auxMessage) : base(String.Format("{0} - {1}", ConstructorNotFoundMessage, auxMessage)) { }
    public ConstructorNotFoundException(string auxMessage, Exception inner) : base(String.Format("{0} - {1}", ConstructorNotFoundMessage, auxMessage), inner) { }
}

InformationsquelleAutor Chris Shepley

Für einen generischen code, der wiederverwendet werden kann, sollten Sie überlegen, thread-Sicherheit an der Stelle, wo Sie erstellen, die die singleton-Instanz.

Als es ist, (instance == null) könnte zu true ausgewertet werden in separaten threads.

InformationsquelleAutor xyz

Diese wie ich es gemacht habe, mit dem Aktuellen Muster (auch thread-sichere Initialisierung)

public static class Singleton<T> 
{
    private static readonly object Sync = new object();

    public static T GetSingleton(ref T singletonMember, Func<T> initializer)
    {
        if (singletonMember == null)
        {
            lock (Sync)
            {
                if (singletonMember == null)
                    singletonMember = initializer();
            }
        }
        return singletonMember;
    }
}

Verwendung:

private static MyType _current;
public static MyType Current = Singleton<MyType>.GetSingleton(ref _current, () => new MyType());

Verbrauchen die singleton:

MyType.Current. ...

InformationsquelleAutor MaurGi