C# – ASP.NET C# Static Variables are global

asp.netcstaticvariables

Today I released a small asp.net beta web application which allows internal staff to modify some product information. We started running into issues where users were overwriting each others product information… even though each staff member was editing a totally different row (Product).

After some searching on google, I think I know what is going on, its to do with the use of static variables, below is a quick rough example of the problem:

// EditProductGroup.aspx.cs
public partial class EditProductGroup : System.Web.UI.Page
{

    private static int _groupId = 0;

    protected void Page_Load(object sender, EventArgs e)
    {
      _groupId = Convert.ToInt16(Request.QueryString["GroupID"]);

     // get existing ProductGroup information from database using 
     //_groupId to find the row and populate textboxes
    }

    private void saveProductGroupData()
    {
      // when user hits 'save changes' button, update row 
      //where the primary key column 'ProductGroupID' matches _groupId in the table
    }
}

So, according to my research, a static variable actually exists to the application as a whole, that means that if multiple users are using the application they will effectively all be reading the same 'value' for '_groupId' and in some cases actually setting it to a different value causing another users 'instance' of the page to save data to the wrong row (ProductGroupId).

My intention was that the static variable is isolated from other users, and should not intefere – each user has their own instance of the page thus their own instance of the '_groupId' variable.

Luckily it was all taking place on a dev/staging database not the live db. I'm not sure whether I just need to drop off the 'static' keyword to stop the variable being set/read by everyone.

Any thoughts? Thanks

Best Answer

Yes, in ASP.NET a static fields lifetime is for the app domain (note that this differs a bit for generic types).

I'd recommend using the server Session for storage of data you want to associate with an instance of a client browser session (user login). i.e.

Session["_groupID"] = Convert.ToInt16(Request.QueryString["GroupID"]);

you can retrieve it by doing:

short groupID = Convert.ToInt16(Session["_groupID"]);

Related Solutions

Python – Are static class variables possible in Python

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass:
...     i = 3
...
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have

>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can't be accessed using a reference to an instance.

See what the Python tutorial has to say on the subject of classes and class objects.

@Steve Johnson has already answered regarding static methods, also documented under "Built-in Functions" in the Python Library Reference.

class C:
    @staticmethod
    def f(arg1, arg2, ...): ...

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I'm still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn't matter.

C# – Are static class instances unique to a request or a server in ASP.NET

Your static classes and static instance fields are shared between all requests to the application, and has the same lifetime as the application domain. Therefore, you should be careful when using static instances, since you might have synchronization issues and the like. Also bear in mind, that static instances will not be GC'ed before the application pool is recycled, and therefore everything that is referenced by the static instance, will not be GC'ed. This can lead to memory usage problems.

If you need an instance with the same lifetime as a request, I would suggest to use the HttpContext.Current.Items collection. This is by design meant to be a place to store stuff that you need througout the request. For nicer design and readability, you can use the Singleton pattern to help you manage these items. Simply create a Singleton class that stores its instance in HttpContext.Current.Items. (In my common library for ASP.NET, I have a generic SingletonRequest class for this purpose).

Best Answer

Related Solutions

Python – Are static class variables possible in Python

C# – Are static class instances unique to a request or a server in ASP.NET

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