Access to disposed closure in C#?

Question

I'm investigating Microsoft enterprise library (data application block) -- The samples sln.

They have a sample of reading data asynchronously ( IAsync , although the new ver (6) also support async).

But Resharper(or visual studio- nevermind) shows me : "Access to disposed closure" : (first i will show the image , so it will be clearer , then i'll paste the code )

code :

/*1*/    [Description("Execute a command that retrieves data asynchronously")] /*2*/    static void ReadDataAsynchronously() /*3*/    { /*4*/        if (!SupportsAsync(asyncDB)) return; /*5*/    /*6*/        using(var doneWaitingEvent = new ManualResetEvent(false)) /*7*/        using(var readCompleteEvent = new ManualResetEvent(false)) /*8*/        { /*9*/            try /*10*/            { /*11*/                // Create command to execute stored procedure and add parameters /*12*/                DbCommand cmd = asyncDB.GetStoredProcCommand("ListOrdersSlowly"); /*13*/                asyncDB.AddInParameter(cmd, "state", DbType.String, "Colorado"); /*14*/                asyncDB.AddInParameter(cmd, "status", DbType.String, "DRAFT"); /*15*/                // Execute the query asynchronously specifying the command and the /*16*/                // expression to execute when the data access process completes. /*17*/                asyncDB.BeginExecuteReader(cmd, /*18*/                    asyncResult = > /*19*/                    { /*20*/                        // Lambda expression executed when the data access completes. /*21*/                        doneWaitingEvent.Set(); /*22*/                        try /*23*/                        { /*24*/                            using(IDataReader reader = asyncDB.EndExecuteReader(asyncResult)) /*25*/                            { /*26*/                                Console.WriteLine(); /*27*/                                Console.WriteLine(); /*28*/                                DisplayRowValues(reader); /*29*/                            } /*30*/                        } /*31*/                        catch (Exception ex) /*32*/                        { /*33*/                            Console.WriteLine("Error after data access completed: {0}", ex.Message); /*34*/                        } /*35*/                        finally /*36*/                        { /*37*/                            readCompleteEvent.Set(); /*38*/                        } /*39*/                    }, null); /*40*/    /*41*/                // Display waiting messages to indicate executing asynchronouly /*42*/                while (!doneWaitingEvent.WaitOne(1000)) /*43*/                { /*44*/                    Console.Write("Waiting... "); /*45*/                } /*46*/    /*47*/                // Allow async thread to write results before displaying "continue" prompt /*48*/                readCompleteEvent.WaitOne(); /*49*/            } /*50*/            catch (Exception ex) /*51*/            { /*52*/                Console.WriteLine("Error while starting data access: {0}", ex.Message); /*53*/            } /*54*/        } /*55*/    } 

Question :

Why is it giving this warning ? there is a manualreset-checked-signal (which runs in a loop) which prevents the using clause to be reached - which means - no dispose will call .

So why does it yell (warning) ?

Answer 1

You pass doneWaitingEvent to a lambda that may extend beyond the scope of the using block. I.e. there's a risk that Dispose will have been called when the lambda executes.

Answer 2

It yells warning because the engine is not smart enough to figure out that the using block will never be exited before the delegate code completes. That is why this is a warning not a error.

You can safely ignore this warning, you can have resharper suppress the warning by wrapping the lines with special comments

asyncDB.BeginExecuteReader(cmd, asyncResult => {     // Lambda expression executed when the data access completes.     // ReSharper disable AccessToDisposedClosure     doneWaitingEvent.Set();     // ReSharper restore AccessToDisposedClosure     try     {         using (IDataReader reader = asyncDB.EndExecuteReader(asyncResult))         {             Console.WriteLine();             Console.WriteLine();             DisplayRowValues(reader);         }     }     catch (Exception ex)     {         Console.WriteLine("Error after data access completed: {0}", ex.Message);     }     finally     {         // ReSharper disable AccessToDisposedClosure         readCompleteEvent.Set();         // ReSharper restore AccessToDisposedClosure     } }, null);