SQLite-Net-PCL Deadlock issue

Question

I am using SQLite-PCL with Xamarin.Android for data storage. I am using it asynchronously, and am experiencing a deadlock issue because of this.

The implementation is contained in a DataHandler class:

Constructor

public DataHandler(string path)
{
      _db = new SQLiteAsyncConnection(path);
      Initialize().Wait();
}

Initialize Function

private async Task Initialize()
{
    using (await Lock())
    {
        await _db.CreateTableAsync<Person>();
        await _db.CreateTableAsync<Animal>();
    }
 }

And lastly, that Lock() function is an implementation of the answer at the question here: https://stackoverflow.com/a/44127898/3808312

When the object is constructed, Initialize().Wait() is called and deadlocks on the first call to CreateTableAsync() and unfortunately, I can't really debug into the library without touching the disassembly of it. Am I using async pattern wrong or something? And yes, I do know that Wait() is synchronous. That was just to keep the same format as the other methods in the class.

It will only deadlock if you capture the Sync-Context (which is done by default with async). You must use ConfigureAwait( false ) on all awaitable calls to state you do not want to capture the Sync-Context. Read blog.stephencleary.com/2012/07/dont-block-on-async-code.html from Stephen Cleary and also all of his other articles about async/await — Sir Rufo
– Sir Rufo, Commented Jun 3, 2017 at 21:32

Nkosi · Accepted Answer · 2017-09-14 10:37:05Z

For issues like this, a frequent pattern is to create the affected class using an async factory method.

public class DataHandler {

    //...other code 

    private DataHandler() {

    }

    private async Task InitializeAsync(string path) {
        _db = new SQLiteAsyncConnection(path);
        using (await Lock()) {
            await _db.CreateTableAsync<Person>();
            await _db.CreateTableAsync<Animal>();
        }
    }

    public static async Task<DataHandler> CreateDataHandler(string path) {
        var handler = new DataHandler();
        await handler.InitializeAsync(path);
        return handler;
    }

    //...other code 
}

and then use it in a manner that allows async calls.

var handler = await DataHandler.CreateDataHandler("<path here>");

Like in the OnAppearing virtual method where you can subscribe to the Appearing event of the page/view

protected override void OnAppearing() {
    this.Appearing += Page_Appearing;
}

and call your async code on an actual even handler

private async void Page_Appearing(object sender, EventArgs e) {
    //...call async code here
    var handler = await DataHandler.CreateDataHandler("<path here>");
    //..do what you need to do with the handler.

    //unsubscribing from the event
    this.Appearing -= Page_Appearing;
}

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