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Singleton Pattern VB.NET using Static Variable

Evgeniy Sukhikh

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3.25/5 (4 votes)

24 May 2012CPOL

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The most elegant implementation for VB.NET and safe as simple

Introduction

I have seen many implementations of the Singleton pattern. This one seems to be the most elegant for VB.NET and is as safe as simple.

Theory

Static Local Variables in VB.NET

In VB, a local variable marked as static, apart from usual local variable, continues to exist and retains its most recent value. Moreover, it is initialized only once and this operation is thread safe.

What it simply means is that while the first time property is accessed, the variable is defined and initialized with a value given. Every next call initialization and declaration is ignored and the variable is used as shared/static for this class and continues to exist for a lifetime of the class.

Singleton Pattern Implementation

VB.NET

Public Class Singleton

    Private Sub New()

    End Sub

    Public Shared ReadOnly Property GetInstance As Singleton
        Get
            Static Instance As Singleton = New Singleton
            Return Instance
        End Get
    End Property

End Class

Please do let me know if there are any pitfalls or if you have other ideas.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By

Evgeniy Sukhikh

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Ukraine

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Comments and Discussions

I think that it is wrong because this singleton is always creating a new instance, you need to question if is this nothing

Nicolás Bjorklund14-Nov-13 6:26

Nicolás Bjorklund

14-Nov-13 6:26

VB

Public Shared ReadOnly Property GetInstance As Singleton
    Get
        Static Instance As Singleton
        If Instance Is Nothing Then
            Instance = New Singleton
        End If
        Return Instance
    End Get
End Property

Re: I think that it is wrong because this singleton is always creating a new instance, you need to question if is this nothing

MPelletierProg9-Apr-14 5:07

9-Apr-14 5:07

I agreed at first, but then saw this:

Normally, a local variable in a procedure ceases to exist as soon as the procedure stops. A static variable continues to exist and retains its most recent value. The next time your code calls the procedure, the variable is not reinitialized, and it still holds the latest value that you assigned to it. A static variable continues to exist for the lifetime of the class or module that it is defined in.

EDIT: But your version would work too.

modified 9-Apr-14 12:26pm.

Re: I think that it is wrong because this singleton is always creating a new instance, you need to question if is this nothing

Evgeniy Sukhikh5-May-17 6:17

Evgeniy Sukhikh

5-May-17 6:17

What you are suggesting is kind of how compiler will do a job!

AspDotNetDev25-May-12 13:40

25-May-12 13:40

You called your class Singletone for some reason. I recommend you change it to Singleton.

Thou mewling ill-breeding pignut!

Evgeniy Sukhikh26-May-12 9:10

Evgeniy Sukhikh

26-May-12 9:10

Fixed

It still needs more explanation

PIEBALDconsult25-May-12 8:39

25-May-12 8:39

It still needs more explanation of how it works. And not just a link to some blog -- give a brief explanation and a link to MSDN.

"
Normally, a local variable in a procedure ceases to exist as soon as the procedure stops. A static variable continues to exist and retains its most recent value. The next time your code calls the procedure, the variable is not reinitialized, and it still holds the latest value that you assigned to it. A static variable continues to exist for the lifetime of the class or module that it is defined in.
"

Something like thishttp://msdn.microsoft.com/en-us/library/z2cty7t8.aspx[^]

Re: It still needs more explanation

Evgeniy Sukhikh26-May-12 9:32

Evgeniy Sukhikh

26-May-12 9:32

Thanks for a constructive suggestion. I wouldn't copy paste from MSDN, though few words have been added with references to sources.
Thanks again. This is my first time.

Ctznkane24-May-12 8:33

24-May-12 8:33

Shouldn't there be a Shared Sub New() call that instantiates the singleton object? Rather than doing it on each Property call.

Re: Instantiation

PIEBALDconsult25-May-12 8:29

25-May-12 8:29

Ctznkane wrote:
Shouldn't there be a Shared Sub New() call that instantiates the singleton
object?

No.

Ctznkane wrote:
Rather than doing it on each Property call.

You need to read up on static variables in VB to understand it.

fjdiewornncalwe23-May-12 8:45

fjdiewornncalwe

23-May-12 8:45

I have a fundamental issue with your sample. You are recreating the variable on each call to the property. Instead, you should have a private static member variable that is checked for nothing before it is created so that it isn't being recreated over and over again. Personally, I find that it defeats the purpose of a singleton if you do it this way.

Re: My vote of 1

Evgeniy Sukhikh23-May-12 11:15

Evgeniy Sukhikh

23-May-12 11:15

Here is a good explanation to static local variable. Please take a loot second part of the article where initialization is explained.

Static construction for local variables is rarely used therefore it confused you a bit.

Re: My vote of 1

kfsdavidt6424-May-12 3:25

24-May-12 3:25

In Visual Basic, a variable marked as Static is just like a private static member variable, but its scope is the enclosing Function or Sub rather than the entire class.

David

Re: My vote of 1

Evgeniy Sukhikh24-May-12 22:57

Evgeniy Sukhikh

24-May-12 22:57

Yes. This is a static variable which is accessible through enclosing block (function, sub, property...) And it is only once initialized. By the way, this example converted to C#:

C#

public class Singletone
{


	private Singletone()
	{
	}
	static readonly Microsoft.VisualBasic.CompilerServices.StaticLocalInitFlag static_GetInstance_Instance_Init = new Microsoft.VisualBasic.CompilerServices.StaticLocalInitFlag();

	static Singletone static_GetInstance_Instance;
	public static Singletone GetInstance {
		get {
			lock (static_GetInstance_Instance_Init) {
				try {
					if (InitStaticVariableHelper(static_GetInstance_Instance_Init)) {
						static_GetInstance_Instance = new Singletone();
					}
				} finally {
					static_GetInstance_Instance_Init.State = 1;
				}
			}
			return static_GetInstance_Instance;
		}
	}
	static bool InitStaticVariableHelper(Microsoft.VisualBasic.CompilerServices.StaticLocalInitFlag flag)
	{
		if (flag.State == 0) {
			flag.State = 2;
			return true;
		} else if (flag.State == 2) {
			throw new Microsoft.VisualBasic.CompilerServices.IncompleteInitialization();
		} else {
			return false;
		}
	}

}

Good, though...

JoseMenendez23-May-12 8:37

23-May-12 8:37

Though that's a pretty good implementation of a singleton, I have found useful to make preferably all methods static, which is easier to use when coding. Microsoft uses this model too, in several cases along the .net framework.

J

Re: Good, though...

Evgeniy Sukhikh23-May-12 11:16

Evgeniy Sukhikh

23-May-12 11:16

Shared methods are static in terms of C#

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