Diff. between singleton class and static class | Bytes (2024)

Hi All,

What is the diff. between a singleton class and a static class in C#?

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A static class does not allow an instance of itself to be created by anything
other than itself - it has no public ctors:

class T
{
private T() { }

public static T Get() { return new T(); }
};

T t1 = new T(); // Error, no public constructor
T t2 = T.Get(); // Ok
T t3 = T.Get(); // Ok; returns another instance of T

A singleton allows only one instance of the class to ever be created:

class S
{
private S() { }

private static S instance = new S();

public static S Get() { return S.instance; }
};

S s1 = new S(); // Error, no public constructor
S s2 = S.Get(); // Ok; always returns the same instnace
S s3 = S.Get(); // Ok; returns the same instance as s2

"DBA" wrote:

Hi All,

What is the diff. between a singleton class and a static class in C#?

DBA <DB*@discussion s.microsoft.com > wrote:

What is the diff. between a singleton class and a static class in C#?

A singleton allows access to a single created instance - that instance
(or rather, a reference to that instance) can be passed as a parameter
to other methods, and treated as a normal object.

A static class allows only static methods.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

Hi,

first the currently there is no a way to declare a class static, you do it
implicitely by declaring static all the members of it. The members will
exist as long as the Appdomain does.

A singleton is a "normal" class with the only difference that the
constructor is private, hence the only way to "create" an instance is using
the property/method that return the instance. If needed you can also declare
a method that dispose the instance, so it can be recreated again when
needed.
cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"DBA" <DB*@discussion s.microsoft.com > wrote in message
news:D4******** *************** ***********@mic rosoft.com...

Hi All,

What is the diff. between a singleton class and a static class in C#?

Ignacio,

The statement:

first the currently there is no a way to declare a class static
Is not true. In the current beta release of C# 2.0, there is support
for static classes, where all of the members must be static (and the
compiler issues a warning if any of the members are not).

--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"Ignacio Machin ( .NET/ C# MVP )" <ignacio.mach in AT dot.state.fl.us > wrote
in message news:uI******** ******@TK2MSFTN GP15.phx.gbl... Hi,

first the currently there is no a way to declare a class static, you do it
implicitely by declaring static all the members of it. The members will
exist as long as the Appdomain does.

A singleton is a "normal" class with the only difference that the
constructor is private, hence the only way to "create" an instance is
using the property/method that return the instance. If needed you can also
declare a method that dispose the instance, so it can be recreated again
when needed.
cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"DBA" <DB*@discussion s.microsoft.com > wrote in message
news:D4******** *************** ***********@mic rosoft.com...

Hi All,

What is the diff. between a singleton class and a static class in C#?

You can declare the entire class as static in VB. I believe the next version
of C# will also have that ability, but I'm not certain.

The only reasons (that I know) to use a singleton is if you need to pass it
to a function call. For example, an IComparer class.
--
Jonathan Allen
"Ignacio Machin ( .NET/ C# MVP )" <ignacio.mach in AT dot.state.fl.us > wrote
in message news:uI******** ******@TK2MSFTN GP15.phx.gbl...

Hi,

first the currently there is no a way to declare a class static, you do it
implicitely by declaring static all the members of it. The members will
exist as long as the Appdomain does.

A singleton is a "normal" class with the only difference that the
constructor is private, hence the only way to "create" an instance is
using the property/method that return the instance. If needed you can also
declare a method that dispose the instance, so it can be recreated again
when needed.
cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"DBA" <DB*@discussion s.microsoft.com > wrote in message
news:D4******** *************** ***********@mic rosoft.com...

Hi All,

What is the diff. between a singleton class and a static class in C#?

Hi,

Yes, I meant that , just that I totally miswrote that sentence :)

cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote in
message news:er******** ******@TK2MSFTN GP14.phx.gbl...

Ignacio,

The statement:

first the currently there is no a way to declare a class static

Is not true. In the current beta release of C# 2.0, there is support
for static classes, where all of the members must be static (and the
compiler issues a warning if any of the members are not).

--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"Ignacio Machin ( .NET/ C# MVP )" <ignacio.mach in AT dot.state.fl.us >
wrote in message news:uI******** ******@TK2MSFTN GP15.phx.gbl...

Hi,

first the currently there is no a way to declare a class static, you do
it implicitely by declaring static all the members of it. The members
will exist as long as the Appdomain does.

A singleton is a "normal" class with the only difference that the
constructor is private, hence the only way to "create" an instance is
using the property/method that return the instance. If needed you can
also declare a method that dispose the instance, so it can be recreated
again when needed.
cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"DBA" <DB*@discussion s.microsoft.com > wrote in message
news:D4******** *************** ***********@mic rosoft.com...

Hi All,

What is the diff. between a singleton class and a static class in C#?

KH, Jon Thank you for responses. One more question:
When to use a singleton class and when to use a staic class? I think both
has the same functionality

"Jon Skeet [C# MVP]" wrote:

DBA <DB*@discussion s.microsoft.com > wrote:

What is the diff. between a singleton class and a static class in C#?

A singleton allows access to a single created instance - that instance
(or rather, a reference to that instance) can be passed as a parameter
to other methods, and treated as a normal object.

A static class allows only static methods.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

=?Utf-8?B?REJB?= <DB*@discussion s.microsoft.com > wrote in
news:BE******** *************** ***********@mic rosoft.com:

KH, Jon Thank you for responses. One more question:
When to use a singleton class and when to use a staic class? I think
both has the same functionality

For one example, whenever you want to be able to pass the instance of the
class as a parameter to a function... Just because you have a
"singleton" class doesn't mean that there's only one singleton class
(although by definition there's only one instance of *each* of them)...
Such as...

public interface IWorkInterface
{
void DoSomeWork();
}

public SingletonA : IWorkInterface
{
.... singleton implementation ...
public void DoSomeWork() { ... }
}

public SingletonB : IWorkInterface
{
.... singleton implementation ...
public void DoSomeWork() { ... }
}

public void StartWork(IWork Interface iwi)
{
iwi.DoSomeWork( );
}

public void Main()
{
IWorkInterface singletonA = new SingletonA();
IWorkInterface singletonB = new SingletonB();

// here i'm passing the object - its a singleton
StartWork(singl etonA);

// here i'm passing a different singleton
StartWork(singl etonB);
}

--
-mdb

First, the Singleton Pattern generally creates a single instance of a
class, but
this is not absolute. In fact, you can argue that some of the
flexibility of the
Singleton Pattern is that it allows you the programmer to return more
than
one instance of a class at a later date without breaking the client
code. The
classic example is an expensive hardware resource. You implement this as
a
Singleton Pattern. Later, when you win the lottery you can purchase a
second
unit of this expensive hardware resource and recode the class factory to
return one of the two hardware controllers using load sharing. The
caller does
not know that there are now two units.

Perhaps it would help to think of where the code goes and how this could
help you understand the difference between static methods and fields and
a
Singleton Pattern.

In a Static call you have one set of static methods and fields in
memory. There
is no need for a pointer to the instance stack frame. Now let's go back
to the
classic Singleton where you have one instance of the class. Although the
code
for instance methods _appears_ to be copied for each instance so that
each
instance has BEHAVIOR, in reality the code for instance methods is
_shared_!
So the only real difference between Static calls and instance calls is
that
instance calls act on instance data and each instance has its own
separate
data in memory. But if you only have one instance then there is only one
copy
of instance data. SO, the only real difference between Static and a
single
instance is that there is a pointer to this to access the instance data.

There _are_ advantages to the Singleton pattern and they include:

1) Adds a level of indirection. This allows the creation of more than
one
instance of the class at a later date without breaking client code.
2) Encapsulates data and methods into a separate namespace, the
singleton
class.
3) Allows sub-classing.
4) Provides access control to the single instance.

Hope that helps.

Have fun storming the castle,
Jeff

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