NullReferenceException problem when loading savegame in XNA 4.0

Question

I want to enable my program to load "savegames", and I followed the tutorial on MSDN, but when I run my code, I get the NullReferenceException error. I use an xml file as my savegame data container, and it works, but at this point I have to rebuild my game every time I make some changes to that xml file, and I want to be able to do it during run-time. The xml file is stored in my Content folder. This is the code of my load method:

   public void Otvori()
        {

            IAsyncResult rezultat = device.BeginOpenContainer("OtvaranjeDemo", null, null);           
            rezultat.AsyncWaitHandle.WaitOne();

            container = device.EndOpenContainer(rezultat);
            rezultat.AsyncWaitHandle.Close();
            if (!container.FileExists(imeDatoteke))
            {
                container.Dispose();
                return;
            }
            Stream stream = container.OpenFile(imeDatoteke, FileMode.Open);
            XmlSerializer serializer = new XmlSerializer(typeof(PostavkeStanice));
            PostavkeStanice postavke = (PostavkeStanice)serializer.Deserialize(stream);
            stream.Close();
            container.Dispose();
        }

The compiler throws me the error at this line "IAsyncResult rezultat = device.BeginOpenContainer("OtvaranjeDemo", null, null);"

Since this is a method in a game library, I thought it had something to do with variable definition scope, but I declared the variable in the game wide scope, and I use this method inside the Update method, so that should not be the problem? Do you have any ideas? Thanks

Answer 1

You need to get a device first using StorageDevice.BeginShowSelector In the Async callback of that method, you can assign the device to your variable. Only then can you call methods on it.
See Getting a StorageDevice Asyncronously (MSDN).

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I suggest using the EasyStorage helper classes that encapsulates a lot of the setup of loading/saving.