The background to this question is that I need to use some user session data in a (static) WebMethod. I have created a static property that references the data I need like so:
private static UserWebSession UserWebSession
{
get
{
return (UserWebSession)HttpContext.Current.Session["UserWebSession"];
}
}
I can then call this in my page's static WebMethod.
My question is, is this technique thread safe? Or will this property's value be updated with every new page request - in other words, it will return the UserWebSession for the user who most recently requested the page?
That's fine - HttpContext.Current is designed precisely for this sort of thing. You won't get a previous user's session.
It's dependent on the thread though (I believe) - so if you start any extra background threads, they won't be able to see the current context.
Also be aware that although this call is safe in terms of not getting the wrong context, the normal concurrency caveats apply when it comes to what you actually do with the context.
I don't know that a Page Method is able to access Session state. If it can, then you may be ok. I recall that access to Session state is serialized, so that only one request at a time can arrive for a given session.
Related
In my application there are sometimes in that I want to get a user request, save it in some place and then, in a next request, simulate saved request instead of real request, is that possible?
Use any web debugger. I use Fiddler 2.6.2 myself.
Find it here, https://www.telerik.com/download/fiddler
Be warned - there is no robust way to do it (there are a lot of edge cases to consider) but hopefully this will get you started:
Take Request.Form.ToString() and save it against the user somewhere. For testing I suggest putting it in a hidden field, but at some point you would probably need to move that to a database and associate that with the user somehow.
You will need to have a field that will tell you whether the previous request should be replayed. Using a query string parameter like ?replay=true will work. Then pull it into your page:
protected bool IsReplayRequest
{
get
{
return bool.Parse(this.GetValue(p => p.Request["replay"], "false"));
}
}
To simulate the request you will need to override the page's DeterminePostBackMode. This is one of the first methods ASP.NET calls to start the postback process. Use the saved request form converted into a NameValueCollection. Something like this:
protected override NameValueCollection DeterminePostBackMode()
{
if (IsReplayRequest && SavedFormData != null)
{
if (!string.IsNullOrWhiteSpace(SavedFormData.Data))
{
return HttpUtility.ParseQueryString(SavedFormData.Data ?? string.Empty);
}
}
return base.DeterminePostBackMode();
}
Where SavedFormData is just a class which holds the request form and the user information.
With those 3 steps, you should be able to succesfully simulate a previous request for a simple page
Now for the potential problems:
First, the viewstate might get in the way. If you have any listboxes or data grids or authentication etc, you will then need to save the viewstate with the form data. for that you will need to override the page GetStatePersister() method (Is it possible to make a custom implementation of ViewState?).
Second, once you have simulated a request, you need to make sure a) no data is corrupted and b) you remove the "replay" query string. Otherwise the request will play over and over again.
Third, depending on how you store the request form, you will have to think about how to associate it with the user/browser/session/window. Dealing with each one of those has it's own problems.
Once you've got everything above solved, you will successfully simulate a saved request!
I just discovered System.Web.Caching.Cache used in a project that I am working on and I am having a hard time finding more information on it.
My question is how this cache is persisted? Is it client-side (similar to ViewState), server-side (Session)? Entirely different?
Example:
protected string FileContent
{
get
{
return Cache[FILE_UPLOAD_KEY + Id] as string ?? GetFileUpload();
}
}
It's a server-side, application-wide cache.
One instance of this class is created per application domain, and it
remains valid as long as the application domain remains active.
Information about an instance of this class is available through the
Cache property of the HttpContext object or the Cache property of the
Page object. (Cache Class, MSDN)
It grants the ability to set time limits and so forth on cached objects. And it doesn't promise the object will be there when you need it again. It keeps items in cache only so long as there is sufficient memory to do so.
So, it's not intended for passing objects between page views (use ViewState or Session for that) or controls (use Items for that). It's intended to cache global objects (accessible in any request from all clients) that are expensive to build.
It's persisted at the server, and it's global across sessions, like Application. So when you set a value in the Cache, it's available to all users until it expires.
EDIT
The example you've got probably isn't quite right (unless GetFileUpload() actually writes to the cache). Generally your calls to cache look something like:
string GetSomeStringFromCache()
{
string someString = Cache[SomeKey] as string;
if (someString == null)
{
someString = GetStringUsingSomeExpensiveFunction();
Cache.Add(SomeKey, someString, /*a bunch of other parameters*/);
}
return someString;
}
This will put it in the Cache if it's not already there, but if it is, it will just use it.
In ASP.NET MVC 2, the lifespan of an entry in the TempDataDictionary was just one HTTP Request.
That translated to setting a value in one request, redirecting, and having access to the same item at the other end of the line. After this the entry would be no longer available, regardless of whether you read the value out of the dictionary at the latter end of the line or not.
Since ASP.NET MVC 3 (I believe), this implementation detail has changed quite significantly.
Entries in the TempDataDictionary are now only removed once they've been read.
MVC 4
public object this[string key]
{
get
{
object obj;
if (!this.TryGetValue(key, out obj))
return (object) null;
this._initialKeys.Remove(key);
return obj;
}
}
and
public bool TryGetValue(string key, out object value)
{
this._initialKeys.Remove(key);
return this._data.TryGetValue(key, out value);
}
MVC 2:
public object this[string key] {
get {
object value;
if (TryGetValue(key, out value)) {
return value;
}
return null;
}
and
public bool TryGetValue(string key, out object value) {
return _data.TryGetValue(key, out value);
}
Since most people seem to put items in the TempData collection in one request and immediately read them back out in the immediate next request, the functionality seems roughtly the same.
In scenarios where this is not the case such as wanting to read the TempData entry if redirected to one place, and expecting it to have been removed if requesting other resources and navigating back, this change has quite an impact.
No longer is the entry available for one http request but is available over many HTTP requests, be it only available to one single get on the dictionary.
I'd like to know more about this implimentation change, what were the reasons for the change, was this simply to cater for multiple redirects or are there deeper benefits?
Secondary to that, I'm intrigued to know if there's anything built in that now caters for single HTTP request sharing of data in the same way that TempData used to cater for?
You're correct that TempData keys are only cleared if they’ve been read (or after the user’s session expires) but this has been the case since MVC2, (http://forums.asp.net/post/3692286.aspx)
I'd like to know more about this implimentation change, what were the
reasons for the change, was this simply to cater for multiple
redirects or are there deeper benefits?
This change prevented problems that arose in MVC 1, such as TempData keys being deleted before they were read. So yes, the primary benefit is in avoiding these problems when you have multiple re-directs, or interleaved requests. In addition, the RedirectToRouteResult or RedirectResult methods now automatically call TempData.Keep() to prevent clearing of keys, even after they've been read so keep that in mind as well.
In scenarios where this is not the case such as wanting to read the
TempData entry if redirected to one place, and expecting it to have
been removed if requesting other resources and navigating back, this
change has quite an impact.
You’re correct, if you've been coding under the assumption that the TempData keys are cleared automatically you could run into unexpected problems. You can call TempData.Clear() to manually remove all keys from the TempDataDictionary, or TempData.Remove(key) to remove a specific key. You can also use TempData.Peek() to read the value of a TempData key without flagging it for removal from the TempDataDictionary.
Secondary to that, I'm intrigued to know if there's anything built in
that now caters for single HTTP request sharing of data in the same
way that TempData used to cater for?
I'm not aware of any new objects or functions that replicate the original implementation of TempData. Essentially we still use TempData but have to be mindful that the data persists until read and clear the dictionary manually if needed.
If I use a protected variable, does the variable exist for the whole web application or does it get removed when the user move to other page by get or post? I do know that it is not accessible in other pages unless I use static variable, but I am curious as to if it exists for the whole application. Please let me know!
when you move to other page and return, a new instance of your page class will be created and so all non static variables will be reset.
The value will be valid in a one request process life time (starts with request start and ends with request end)
making a variable protected, just means that this variable is access-able in inherited class. for example in asp.net you can use it in inherited class like inside your markup (because it inherits code behind class)
this is the meaning of protected variable
if you want to keep a value saved between pages you can use one of these items depending on your requirement :
Cookie
Query String
Session States
Application States
Cache
and ViewState keeps state variable between postback in a same page or control while it is not redirected to another page.
protected keyword does not determine how long a variable exists nor does it determine whether it will be available in the next post back.
What you are probably looking for is state management.
Take a look at this webpage to see how you can maintain state between post backs, different pages etc.
Also take a look at this page to determine which state management feature to use in which situation.
In general, "page" variables only live through the duration of the request. If your variable is static, there will only be one instance of the variable for all the requests until the app domain unloads.
If your variable is private or protected, no other classes will have access to it.
Your question, however, seems a little strange. What's your concern?
I have an ASP.NET MVC application. I have come to an idea of generating autoincremented values to be used as unique element ids. The question is, how can I have and work with a global variable which should be there for the duration of a request (page generation) but no longer?
I thought of using TempData for this shared variable and then just delete this key when the page is done. But then, where in code to purge this TempData key? Obviously it has to be some very last piece of code where the page has been rendered already.
Any input is highly appreciated.
EDIT: I have a number of HTML helpers that can be called from various views and partial views, so declaring a variable on a page and passing it to each helper is obviously not a good solution. I wish to just use the helpers and know they all are getting unique ids behind the scenes.
Okay, I have googled a little bit and found a solution on ASP.NET forums.
http://forums.asp.net/t/1401685.aspx
Obviously, I can use the HttpContext.Current.Items collection to have my little static variable for the duration of a request.
If all you need is to store a number, the resources that would take to manage its lifestyle would take a lot more than just having a one static integer and always reusing it.
Do not bother deleting the key after each request. Just use a static (I think this is shared in visual basic) integer, use and increment it every time you need a unique value. Also take its mod with a ridiculously high number each time to make sure it will not be reused in a single request and it will never overflow.
Why don't you define your integer variable at the top of the page view file?
Use it throughout the view rendering execution and at the end of it you can easily leave it as is. You don't have to explicitly destroy anything. Your variables live for the duration of request only. IIS is stateless service (if you subtract Session, Cache and Application variables) so it doesn't really remember anything explicitly.
I would imagine you could use the Application_BeginRequest and Application_EndRequest methods in global.asax.cs; Note I can't double check the method names currently, but I think they are close.
You could create a member variable in your controller which would be regenerated for each request:
public class ItemController : Controller
{
private int _UniqueID = 0;
public ActionResult Index()
{
foreach (var item in items)
{
item.UniqueID = _UniqueID++;
}
// etc...
}