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POWriting to a read only session in MVC 3+
 

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    <p>I've come across a curious behaviour of ASP sessions. You can force a controller to be outside of the user's session - I want to be able to do this so that multiple requests can execute at the same time and using a session causes them to execute consecutively.</p> <p>Disabling session state works as expected:</p> <pre><code>[SessionState(SessionStateBehavior.Disabled)] public class SampleController : Controller { public ActionResult Test() { // Access to the session should be denied object test = Session["test"]; return Content(test); } } </code></pre> <p>Going to <em>~/Sample/Test</em> will throw a <code>System.Web.HttpException</code>, as expected. However, read-only sessions appear to behave a little strangely:</p> <pre><code>[SessionState(SessionStateBehavior.ReadOnly)] public class SampleController : Controller { public ActionResult Test() { // Read from the session should be fine object test = Session["test"]; return Content(test); } public ActionResult SetTest(string value) { // Write to the session should fail Session["test"] = value; // Read it back from the session object test = Session["test"]; return Content(test); } } </code></pre> <p>So now I expect <em>~/Sample/Test</em> to work, and it does. The odd bit is that the set does too: I go to <em>~/Sample/SetTest?value=foo</em> and it doesn't throw an exception, in fact it returns "foo". If I call <em>~/Sample/SetTest?value=bar</em> and then <em>~/Sample/Test</em> I get "bar", indicating that the session has been written to.</p> <p>So in a <code>SessionStateBehavior.ReadOnly</code> I have successfully written to the session and read my value back.</p> <p>I think this could be due to one of three things:</p> <ul> <li>In MVC 3 <code>[SessionState(SessionStateBehavior.ReadOnly)]</code> is broken/ignored.</li> <li>The <code>[SessionState]</code> is overridden when the session is written to and becomes writeable.</li> <li>The <code>SessionStateBehavior.ReadOnly</code> actually indicates some kind of dirty/optimistic access.</li> </ul> <p>Can anyone confirm?</p> <p>I suspect the last one is true, based off the <a href="http://msdn.microsoft.com/en-us/library/ms178587.aspx">custom session provider documentation</a> - if it is then how does the implementation work? Does writing to a 'read only' session risk concurrency errors (i.e. last write wins) or does it risk corrupt sessions and breaking exceptions?</p> <p><strong>Update</strong></p> <p>It looks like this is by design (from <a href="http://msdn.microsoft.com/en-us/library/system.web.sessionstate.sessionstatestoreproviderbase.aspx">Microsoft's docs</a>):</p> <blockquote> <p>Note that even if the EnableSessionState attribute is marked as ReadOnly, other ASP.NET pages in the same application might be able to write to the session store, so a request for read-only session data from the store might still end up waiting for locked data to be freed.</p> </blockquote> <p>It looks like the second option above is what it actually does - the session is locked and the mode changed to writable.</p>
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