Just want to make sure I am not assuming something foolish here, when implementing the singleton pattern in an ASP .Net web application the static variable scope is only for the current user session, right? If a second user is accessing the site it is a different memory scope...?
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7The short answer is no, you're wrong. – Klaus Byskov Pedersen Jan 25 '10 at 18:20
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2Duplicate: http://stackoverflow.com/questions/194999/are-static-class-instances-unique-to-a-request-or-a-server-in-asp-net – Dan Herbert Jan 25 '10 at 19:17
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Static members have a scope of the current worker process only, so it has nothing to do with users, because other requests aren't necessarily handled by the same worker process.
- In order to share data with a specific user and across requests, use HttpContext.Current.Session.
- In order to share data within a specific request, use HttpContext.Current.Items.
- In order to share data across the entire application, either write a mechanism for that, or configure IIS to work with a single process and write a singleton / use Application.
By the way, the default number of worker processes is 1, so this is why the web is full of people thinking that static members have a scope of the entire application.
Moshe Bixenshpaner
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The static variable scope is for the entire app domain, which means other sessions also have access to it. Only if you have a farm with different servers you would have more than one instance of the variable.
Otávio Décio
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4+1 "Only if you have a farm with different servers you would have more than one instance of the variable". – Jim Aho Jul 26 '16 at 12:20
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As others have mentioned, a static variable is global to the entire application, not single requests.
To make a singleton global to only individual requests, you can use the HttpContext.Current.Items dictionary.
public class Singleton
{
private Singleton() { }
public static Singleton Instance
{
get
{
if (HttpContext.Current.Items["yourKey"] == null)
HttpContext.Current.Items["yourKey"] = new Singleton();
return (Singleton)HttpContext.Current.Items["yourKey"];
}
}
}
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Dan Herbert
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Thanks, What is the overhead of this? will it be much slower if I am accessing the singleton quite often? – Wesly Jan 25 '10 at 18:40
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@Ws You shouldn't notice any performance issues with this approach. The dictionary implementation is pretty efficient so it won't slow your app down even if you access it a lot. – Dan Herbert Jan 25 '10 at 19:16
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why should we use "HttpContext.Current.Items" instead of "Session"? – alansiqueira27 Oct 09 '14 at 13:09
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@Seva The lifetime of HttpContext.Current.Items is the current request. ie. 1 request. HttpContext.Current.Session spans the entire session. ie. multiple requests that are part of the same session. – Haohmaru Nov 13 '14 at 14:52
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That is very nice and exactly what I am looking for. Is there some type save aproach availbe 5 years later? – Christian Gollhardt Sep 17 '15 at 18:03
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If you need it to be user or session based then check out the following link. Otherwise, as Otavio said, the singleton is available to the entire domain.
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2@Michael Lang: Yes it was, looks like sam changed his content engine. Updated the link with the new reference. – NotMe Jul 14 '11 at 16:51
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The singleton is used for the entire Application Domain, if you want to store user session-related data, use HttpContext Session which is designed for that purpose. Of course, you probably have to redesign your class structure to be able to come up with a key-value-pair way of dealing with the data you're trying to work with.
Tomas Vana
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