Singleton Pattern with non-default constructor

[Tom]

In a web-application I need to read some configuration from a database.
I like to write a configuration class which will be implemented as
thread-safe singleton.

I connect through ODBC to the database. So to be able to do that I need
to pass the Datasource Name.

How can I create a singleton that has a constructor with a parameter?
Standard singleton pattern always use the default constructor. To
provide an init(string datasource_name ) method is no real fun, since the
state of the singleton instance needed to be checked whenever used.

Is there any better solution to this?

Tom

[=?ISO-8859-1?Q?Arne_Vajh=F8j?=]

Re: Singleton Pattern with non-default constructor

Tom wrote:

In a web-application I need to read some configuration from a database.
I like to write a configuration class which will be implemented as
thread-safe singleton.
>
I connect through ODBC to the database. So to be able to do that I need
to pass the Datasource Name.
>
How can I create a singleton that has a constructor with a parameter?
Standard singleton pattern always use the default constructor. To
provide an init(string datasource_name ) method is no real fun, since the
state of the singleton instance needed to be checked whenever used.
>
Is there any better solution to this?

From the philosophical point of view you could arhue that if the
parameter is not fixed, then it is not really a singleton.

From the practical point of view you would probably just have the
constructor read the name from a config file.

The alternative would be a multipleton, where the GetInstance
method has a string argument and the static instance field
is a Dictionary<stri ng,Xand the GetInstance lookup and create
if necessary.

Arne

[Bob Powell [MVP]]

Re: Singleton Pattern with non-default constructor

If the object returned is state-dependent on the constructor parameter then
this is not a good target for the singleton pattern.

Effectively, the singleton objects state will change for all objects that
have a reference to it.

In this case, and if you really want to use a singleton, a more correct
method would be to maintain the pattern as described by the pattern and
provide a method to set the state.

--
--
Bob Powell [MVP]
Visual C#, System.Drawing

Ramuseco Limited .NET consulting

http://www.ramuseco.com

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"Tom" <dev@bednarz.ch wrote in message
news:ulPu5Fg2IH A.3968@TK2MSFTN GP04.phx.gbl...

In a web-application I need to read some configuration from a database. I
like to write a configuration class which will be implemented as
thread-safe singleton.
>
I connect through ODBC to the database. So to be able to do that I need to
pass the Datasource Name.
>
How can I create a singleton that has a constructor with a parameter?
Standard singleton pattern always use the default constructor. To provide
an init(string datasource_name ) method is no real fun, since the state of
the singleton instance needed to be checked whenever used.
>
Is there any better solution to this?
>
Tom