CSharp/C#, Subclassing Singleton

Mark A. Richma
#16 / 24

Subclassing Singleton

How would I go about creating a Singleton class that can be subclassed?
Specifically, I want to avoid the GoF solution of a private registry of
subclasses, simply because I don't want to have to edit the base class
every time I create a subclass. The registry-based solution maintains
a hash of all known subclasses. To me, this breaks the encapsulation
of the solution.

Is there a way to do this via reflection/introspection:

public class BaseSingleton
{
...
protected Instance()
{
// return unique instance of subclass
???
}

Quote:

}

public class MySingleton : BaseSingleton
{
???

Quote:

}

Sun, 17 Apr 2005 23:30:04 GMT

Chris
#17 / 24

Subclassing Singleton

Here's a conceptual workup of using a BaseSingleton. However,
you'll have to note the limitations on your code with this. Depending
on what you need, it may suffice.

//**************************
public abstract class BaseSingleton
{
protected static BaseSingleton instance;
protected BaseSingleton()
{
}
public static BaseSingleton Instance
{
get { return instance; }
}

Quote:

}

//**************************
public class FooSingleton : BaseSingleton
{
static FooSingleton()
{
instance = new FooSingleton();
}
protected FooSingleton()
{
}
public static FooSingleton DefinedInstance
{
get { return instance; }
}

Quote:

}

//**************************
public class BarSingleton : BaseSingleton
{
static BarSingleton()
{
instance = new BarSingleton();
}
protected BarSingleton()
{
}
public static BarSingleton DefinedInstance
{
get { return instance; }
}

Quote:

}

//**************************

Chris R.

Quote:

> Great! Thanks. I guess what I was trying to accomplish was to
"pull-up"
> the instance member, Instance property, and 'get' logic into the base
class.

> I'm still curious if this is possible ;)

> - Mark

Mon, 18 Apr 2005 02:45:03 GMT

Chris
#18 / 24

Subclassing Singleton

The more I thought about this, the more I believe you need to split
the Data out of your class and create a class of static fields and
methods, rather than putting that in your base class.
This seems to be a more logical design from what I can perceive of
what you're trying to accomplish.

public class DataObject
{
protected static SqlConnection sqlConnector;
protected static DAOSet dataSet1;
static DataObject()
{
//sqlConnector = //code
//dataSet1 = //code
}
public static SqlConnection SqlConnector
{
get {return sqlConnector; }
}
public static DAOSet DataSet1
{
get {return dataSet1; }
}

Quote:

}

Chris R.

Quote:

> Great! Thanks. I guess what I was trying to accomplish was to
"pull-up"
> the instance member, Instance property, and 'get' logic into the base
class.

> I'm still curious if this is possible ;)

> - Mark

Mon, 18 Apr 2005 03:35:52 GMT

Chris
#19 / 24

Subclassing Singleton

Ouch, I knew that and I still made the same mistake...
"static members are not inherited"

But heck, once I saw what Mark was trying to do, I could see a realistic
solution that wasn't so mentally convoluted. Keep things simple (if
possible).

Chris R.

Quote:

> > Here is my solution -- I invite all to improve upon it.

> <snip>

> As soon as you've got another class, you're going to run into
problems -
> they'll both try to register the same instance, so you'll only
actually
> be able to get one of them. I thought the idea was that you could have
> one of *each* of the subclass instances, not one in total? Currently
if
> you have Supplier in the same way as Customer, then one of
> Customer.Instance or Supplier.Instance will throw a
ClassCastException,
> depending on which was constructed first. It strikes me you haven't
> really defined what BaseSingleton.Instance should actually return, in
the
> case where you've got more than one subclass.

> I would also personally make everything private and have protected
> properties to access the resources.

> --
> Jon Skeet

> If replying to the group, please do not mail me at the same time

Mon, 18 Apr 2005 05:32:07 GMT

Chad Myer
#20 / 24

Subclassing Singleton

Hrm. What you might do is have a base class that has only
a protected constructor which provides all these commonalities.

Then, each subclass will provide it's own singleton stuff
(Instance property, etc).

For example:

public DAOSingletonBase
{
protected DAOSingletonBase() {}

protected SqlConnection GetSqlConnection()...

/* yadda, yadda, yadda */

Quote:

}

public MySingleton
{
private static MySingleton instance;

protected MySingleton() : base() {}

public MySingleton Instance
{
get
{
if( instance == null )
MySingleton.instance = new MySingleton();

return MySingleton.instance;
}
}

/* yadda, yadda, yadda */

Quote:

}

That might work better for you.

-c

Quote:

> My requirements are self-imposed :) Here is my idea:

> I have a client-server app in which I have a data
> access object for each class of business object. Each of these
> DAO's has the same SQL connection logic, protected DataSet member,
> etc. There is no need for more than one instance of each of these
> on the client. Hence, the Singleton requirement.

> The idea was to limit the instances to one by using a Singleton,
> and to refactor the redundant logic out of the DAO's by
> pulling it up to a base class.

> My thought was that this base class should be a Singleton, and
> subclasses of it would be responsible for business object specific
> logic, i.e. SQL queries. Each subclass would then inherit the
> connection logic, protected DataSet member, etc. from the base
> class, and still be limited to a single instance each.

> Maybe this clears it up ;)

> Thanks again,
> Mark

> > How would I go about creating a Singleton class that can be
subclassed?
> > Specifically, I want to avoid the GoF solution of a private registry
of
> > subclasses, simply because I don't want to have to edit the base
class
> > every time I create a subclass. The registry-based solution
maintains
> > a hash of all known subclasses. To me, this breaks the encapsulation
> > of the solution.

> > Is there a way to do this via reflection/introspection:

> > public class BaseSingleton
> > {
> > ...
> > protected Instance()
> > {
> > // return unique instance of subclass
> > ???
> > }
> > }

> > public class MySingleton : BaseSingleton
> > {
> > ???
> > }

Mon, 18 Apr 2005 01:39:34 GMT

Mark A. Richma
#21 / 24

Subclassing Singleton

Here is my solution -- I invite all to improve upon it.

Thanks,
Mark

using System;
using System.Data;
using System.Data.SqlClient;

namespace Test
{
public abstract class BaseSingleton
{
protected static BaseSingleton instance;
protected static readonly string credentials = "server=localhost;Initial Catalog=Northwind;uid=sa;pwd=";
protected SqlConnection con = new SqlConnection(credentials);
protected SqlDataAdapter da = new SqlDataAdapter();
protected DataSet ds = new DataSet();

protected BaseSingleton(){}

protected static BaseSingleton Instance
{
get { return instance; }
}
}

public class Customer : BaseSingleton
{
SqlCommand cmdSelect = null;

static Customer()
{
instance = new Customer();
}

protected Customer()
{
cmdSelect = new SqlCommand("SELECT CustomerID, CompanyName FROM Customers", con);
cmdSelect.CommandTimeout = 30;
da.SelectCommand = cmdSelect;
}

public static new Customer Instance
{
get { return (Customer)instance; }
}

public DataSet GetCustomers()
{
con.Open();
da.Fill(ds, "Customers");
con.Close();
return ds;
}
}

public class Test {
public static void Main() {
Customer c = Customer.Instance;
DataSet ds = c.GetCustomers();
}
}

Quote:

}

Mon, 18 Apr 2005 03:51:32 GMT

Mark A. Richma
#22 / 24

Subclassing Singleton

Boy, it's so simple once you have the answer!

One last question: When does "static Customer() {...}" get called? When the
application is instantiated? When the class that references it is
instantiated? Or when Customer.Instance() is called?

I guess that was four questions...but I hope you catch my drift.

Thanks,
Mark

Quote:

> I have to ask, why not just use a has-a instead of an is-a solution
> for this? Now, your Customer can have all fields and methods static.

> //***********************
> public class DataContainer
> {
> protected static readonly string credentials =
> "server=localhost;Initial Catalog=Northwind;uid=sa;pwd=";
> protected SqlConnection con = new SqlConnection(credentials);
> protected SqlDataAdapter da = new SqlDataAdapter();
> protected DataSet ds = new DataSet();

> public SqlDataAdapter DA
> {
> get
> {
> return da;
> }
> }
> public DataSet DS
> {
> get
> {
> return ds;
> }
> }
> public SqlConnection Con
> {
> get
> {
> return con;
> }
> }
> }

> //***********************
> public class Customer
> {
> static DataContainer dataCon;
> static SqlCommand cmdSelect = null;

> static Customer()
> {
> dataCon = new DataContainer();
> cmdSelect = new SqlCommand("SELECT CustomerID, CompanyName FROM
> Customers", dataCon.Con);
> cmdSelect.CommandTimeout = 30;
> dataCon.DA.SelectCommand = cmdSelect;
> }

> public static DataSet GetCustomers()
> {
> dataCon.Con.Open();
> dataCon.DA.Fill(dataCon.DS, "Customers");
> dataCon.Con.Close();
> return dataCon.DS;
> }
> }

> //***********************
> public class Test
> {
> public static void Main()
> {
> DataSet ds = Customer.GetCustomers();
> }
> }

> > Here is my solution -- I invite all to improve upon it.

> > Thanks,
> > Mark

> > using System;
> > using System.Data;
> > using System.Data.SqlClient;

> > namespace Test
> > {
> > public abstract class BaseSingleton
> > {
> > protected static BaseSingleton instance;
> > protected static readonly string credentials =
> "server=localhost;Initial Catalog=Northwind;uid=sa;pwd=";
> > protected SqlConnection con = new SqlConnection(credentials);
> > protected SqlDataAdapter da = new SqlDataAdapter();
> > protected DataSet ds = new DataSet();

> > protected BaseSingleton(){}

> > protected static BaseSingleton Instance
> > {
> > get { return instance; }
> > }
> > }

> > public class Customer : BaseSingleton
> > {
> > SqlCommand cmdSelect = null;

> > static Customer()
> > {
> > instance = new Customer();
> > }

> > protected Customer()
> > {
> > cmdSelect = new SqlCommand("SELECT CustomerID, CompanyName FROM
> Customers", con);
> > cmdSelect.CommandTimeout = 30;
> > da.SelectCommand = cmdSelect;
> > }

> > public static new Customer Instance
> > {
> > get { return (Customer)instance; }
> > }

> > public DataSet GetCustomers()
> > {
> > con.Open();
> > da.Fill(ds, "Customers");
> > con.Close();
> > return ds;
> > }
> > }

> > public class Test {
> > public static void Main() {
> > Customer c = Customer.Instance;
> > DataSet ds = c.GetCustomers();
> > }
> > }

> > }

Mon, 18 Apr 2005 06:35:41 GMT

Chris
#23 / 24

Subclassing Singleton

I have to ask, why not just use a has-a instead of an is-a solution
for this? Now, your Customer can have all fields and methods static.

//***********************
public class DataContainer
{
protected static readonly string credentials =
"server=localhost;Initial Catalog=Northwind;uid=sa;pwd=";
protected SqlConnection con = new SqlConnection(credentials);
protected SqlDataAdapter da = new SqlDataAdapter();
protected DataSet ds = new DataSet();

public SqlDataAdapter DA
{
get
{
return da;
}
}
public DataSet DS
{
get
{
return ds;
}
}
public SqlConnection Con
{
get
{
return con;
}
}

Quote:

}

//***********************
public class Customer
{
static DataContainer dataCon;
static SqlCommand cmdSelect = null;

static Customer()
{
dataCon = new DataContainer();
cmdSelect = new SqlCommand("SELECT CustomerID, CompanyName FROM
Customers", dataCon.Con);
cmdSelect.CommandTimeout = 30;
dataCon.DA.SelectCommand = cmdSelect;
}

public static DataSet GetCustomers()
{
dataCon.Con.Open();
dataCon.DA.Fill(dataCon.DS, "Customers");
dataCon.Con.Close();
return dataCon.DS;
}
}

//***********************
public class Test
{
public static void Main()
{
DataSet ds = Customer.GetCustomers();
}

Quote:

}

Quote:

> Here is my solution -- I invite all to improve upon it.

> Thanks,
> Mark

> using System;
> using System.Data;
> using System.Data.SqlClient;

> namespace Test
> {
> public abstract class BaseSingleton
> {
> protected static BaseSingleton instance;
> protected static readonly string credentials =

"server=localhost;Initial Catalog=Northwind;uid=sa;pwd=";

Quote:

> protected SqlConnection con = new SqlConnection(credentials);
> protected SqlDataAdapter da = new SqlDataAdapter();
> protected DataSet ds = new DataSet();

> protected BaseSingleton(){}

> protected static BaseSingleton Instance
> {
> get { return instance; }
> }
> }

> public class Customer : BaseSingleton
> {
> SqlCommand cmdSelect = null;

> static Customer()
> {
> instance = new Customer();
> }

> protected Customer()
> {
> cmdSelect = new SqlCommand("SELECT CustomerID, CompanyName FROM
Customers", con);
> cmdSelect.CommandTimeout = 30;
> da.SelectCommand = cmdSelect;
> }

> public static new Customer Instance
> {
> get { return (Customer)instance; }
> }

> public DataSet GetCustomers()
> {
> con.Open();
> da.Fill(ds, "Customers");
> con.Close();
> return ds;
> }
> }

> public class Test {
> public static void Main() {
> Customer c = Customer.Instance;
> DataSet ds = c.GetCustomers();
> }
> }

> }

Mon, 18 Apr 2005 04:59:00 GMT

Jon Skee
#24 / 24

Subclassing Singleton

Quote:

> Here is my solution -- I invite all to improve upon it.

<snip>

As soon as you've got another class, you're going to run into problems -
they'll both try to register the same instance, so you'll only actually
be able to get one of them. I thought the idea was that you could have
one of *each* of the subclass instances, not one in total? Currently if
you have Supplier in the same way as Customer, then one of
Customer.Instance or Supplier.Instance will throw a ClassCastException,
depending on which was constructed first. It strikes me you haven't
really defined what BaseSingleton.Instance should actually return, in the
case where you've got more than one subclass.

I would also personally make everything private and have protected
properties to access the resources.

--
Jon Skeet

If replying to the group, please do not mail me at the same time

Mon, 18 Apr 2005 04:45:23 GMT