Realm accessed from incorrect thread

[dharamhbtik]

I am trying to refresh realm through background but it is giving exception.
here is my code :

public static void main(){

//StartRefreshingData is called from here without any thread or async call
}
public class DataManager
{
Realms.Realm _realm;
public DataManager(Func<Realm> realmDel){
_realm = realmDel();
}
private void StartRefrshingData(){
var  _worker = new Thread(RefreshData) { IsBackground = true };
 _worker.Start();
}

1 private async void RefreshData()
2 {
3           using (var realm= Realms.Realm.GetInstance(_realm.Config))
4            {
5                   _ =  realm.Refresh(); // explicit refresh only!
6                   var persons = realm.All<Person>();
7           }
8 }
}

When I am executing above code, it is always throwing exception

Line 5 is giving below error :

[0:] 14:33:31.388 Exception Realms.Exceptions.RealmException: Realm accessed from incorrect thread.
  at Realms.NativeException.ThrowIfNecessary () [0x00009] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Native\NativeException.cs:77 
  at Realms.SharedRealmHandle.Refresh () [0x00008] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Handles\SharedRealmHandle.cs:554 
  at Realms.Realm.Refresh () [0x00006] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Realm.cs:979 
  at  PersonManager.DataManager.RefreshData() [0x000a4] in.....

Line 6 is throwing below error :

[0:] 14:14:43.430 Exception Realms.Exceptions.RealmException: Realm accessed from incorrect thread.
  at Realms.NativeException.ThrowIfNecessary () [0x00009] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Native\NativeException.cs:77 
  at Realms.SharedRealmHandle.CreateResults (Realms.Native.TableKey tableKey) [0x0000f] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Handles\SharedRealmHandle.cs:710 
  at Realms.RealmResults`1[T]..ctor (Realms.Realm realm, Realms.Metadata metadata) [0x00000] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Linq\RealmResults.cs:56 
  at Realms.Realm.All[T] () [0x00059] in D:\a\realm-dotnet\realm-dotnet\Realm\Realm\Realm.cs:1026 
  at PersonManager.DataManager.RefreshData() [0x000c6] in ...... 

As per the documentation, from https://www.mongodb.com/docs/realm-sdks/dotnet/latest/reference/Realms.Realm.html#Realms_Realm_Refresh we have to call the Refresh method in background thread
Remarks
On worker threads (where the SynchronizationContext) is null, this will call the blocking Refresh() method instead. On the main thread (or other threads that have SynchronizationContext), this will wait until the instance automatically updates to resolve the task. Note that you must keep a reference to the Realm until the returned task is resolved.

Any way to prevent this incorrect thread exception?

[nirinchev]

Interesting, I have 3 comments here:

1. The code you posted seems correct to me, so I don't have an obvious explanation for why you would get this exception.
2. Why is your RefreshData method async void if it's not using any awaits? Is it a leftover or did you remove some actual async code in there?
3. Why are you calling Refresh on a background thread? What are you trying to achieve?

[dharamhbtik]

2. Yes, I removed some code which actually do some other operation and are async calls.
3. This Refresh method runs every 3 seconds and checks if there is any update in the localDb then sends it over API.

[nirinchev]

Okay, so the problem is likely related to the async calls - what happens is roughly this:

async Task RefreshData()
{
    // Thread A - we open the Realm
    using var realm = Realm.GetInstance(_realm.Config);
    _ =  realm.Refresh(); // explicit refresh only!
    var persons = realm.All<Person>();

    await DoSomethingAsync();

    // Continuation after the await is scheduled on Thread B

    realm.Refresh(); // Accessing realm on Thread B
}

If you want to do check for Realm changes in the background, it's better to install a synchronization context and rely on notifications. Something like (relies on Nito.AsyncEx.Context):

private void StartRefrshingData()
{
    var cts = new CancellationTokenSource();
    
    // you may want to return the cancellation token source or expose a different way to
    // cancel the background processing.
    Task.Run(() => RefreshData(cts.Token));
}


private void RefreshData(CancellationToken token)
{
    AsyncContext.Run(async () =>
    {
        using var realm = Realm.GetInstance(_realm.Config);

        using var notificationToken = realm.All<Person>().SubscribeForNotifications((sender, changes) =>
        {
            // Do some processing of the changes
            var json = payload.ToJson();
            
             // We don't await the API request. This is business needs specific - if you need
             // to ensure the previous request completed before sending a new one you could 
             // force the async method to execute synchronously by calling `.Wait()`.
            _ = NotifyAPI(json);
        });

        // Wait indefinitely or until cancellation is requested
        var tcs = new TaskCompletionSource();
        token.Register(() => tcs.TrySetResult());

        await tcs.Task;
    });
}

private async Task NotifyAPI(string json)
{
    // Send the json to your API.
}

Note that with the proposed approach you don't need to continuously refresh the background Realm as we've now installed a SynchronizationContext (by wrapping our code into an AsyncContext.Run) which means Realm will notify you as soon as changes are made. If you want to bundle those changes together rather than sending them 1 by 1, you could introduce a simple throttling mechanism.