Nicolas Penin

Hello everybody,

A pattern we regularly find is the singleton pattern. What I suggest you is a .NET implementation. This one deals with a singleton context dependent (singleton by ASP.NET context, by WCF context, by thread, for the whole application)

public class Singleton<T>
{
private Singleton()
{
if (CreatingInstance != null)
CreatingInstance(this, EventArgs.Empty);
}

private static T instance;

[ThreadStatic]
private static T threadInstance;

private static object instanceLock = new object();

public static T Instance
{
get
{
//ASP.NET case
T instance;
if (HttpContext.Current != null)
{
if (UseContextInstance)
instance = (T)HttpContext.Current.Items[typeof(T).AssemblyQualifiedName];
else
instance = (T)HttpContext.Current.Application[typeof(T).AssemblyQualifiedName];
if (instance == null)
{
instance = new Singleton<T>().SingletonInstance;
if (UseContextInstance)
HttpContext.Current.Items[typeof(T).AssemblyQualifiedName] = instance;
else
HttpContext.Current.Application[typeof(T).AssemblyQualifiedName] = instance;
}
return instance;
}
if (OperationContext.Current != null)
{
throw new NotImplementedException();
if (instance == null)
{
HttpContext.Current.Items[typeof(T).AssemblyQualifiedName] = new Singleton<T>().SingletonInstance;
}
return instance;
}
//In case of no HttpContext nor OperationContext
if (UseContextInstance)
{
if (Singleton<T>.threadInstance == null)
Singleton<T>.threadInstance = new Singleton<T>().SingletonInstance;
}
else
{
if (Singleton<T>.instance == null)
{
lock (instanceLock)
{
if (Singleton<T>.instance == null)
Singleton<T>.instance = new Singleton<T>().SingletonInstance;
if (Singleton<T>.instance == null)
TryActivate(out Singleton<T>.instance);

}
}
}
return Singleton<T>.instance;
}
}

private static void TryActivate(out T p)
{
try
{
p = Activator.CreateInstance<T>();
}
catch (MissingMethodException) { p = default(T); }
}

public T SingletonInstance { get; set; }

public static event EventHandler CreatingInstance;

/// <summary>
/// Set this property to <see cref="true" /> to have your singleton dependant on your context :
/// - one per thread in Windows Forms/WPF
/// - one per Context in ASP.NET
/// </summary>
public static bool UseContextInstance { get; set; }
}