HttpUnhandledException ASP.NET

I have a ASP.NET website. If I make a request for a page it works most of the times, but sometimes I get an HttpUnhandledException.

I have tried to log the errors, but from the errors messages I'm not able to solve the problem.

StackTrace:

at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.default_aspx.ProcessRequest(HttpContext context) in c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\4e215a3c\72ef69da\App_Web_ylvnbciw.6.cs:line 0
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Data:

System.Collections.ListDictionaryInternal

BaseException:

System.InvalidOperationException: The connection was not closed. The connection's current state is open.
   at DbCategory.getParentCategories()
   at _Default.Page_Load(Object sender, EventArgs e)
   at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
   at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)
   at System.Web.UI.Control.OnLoad(EventArgs e)
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

TargetSite:

Boolean HandleError(System.Exception)

I have idea that it's something about my session og get variables, but i'm not sure about that. Does anybody have an idea about what it could be?

Solution

Such errors are not because of wrong disposing - The connection was NOT closed.

Such error can be then you use SqlConnection in the static class or method which holds open connection, and after that - in not-static, and in this moment exception can occur.

Update: Well, this method is not very save - it do exactly that I was talking about - he creates connection, but never releases it. SqlConnection is very heavy object, and you should dispose it right after you get or set your data.

You should rewrite your logic - create one connection per page or per operation, and you should never store the connection in your classes.

If this is impossible for you, use the lock statement for ensure singleton, like this:

private static object singleton;

lock (singleton)
{
    // Some manipulations with your server
}