Refactoring api calls to use await

Question

want to update a C# Winforms application to use await. The application calls the MYOB Accountright API via an SDK. I am using Dot Net Framework 4.5.1

The old code is like this

public void GetItems(  CompanyFile companyFile )
        {
            var itemSvc = new ItemService(MyConfiguration, null, MyOAuthKeyService);
            string pageFilter = string.Format("$top={0}&$skip={1}&$orderby=Date desc", PageSize,
                                              PageSize * (_currentPage - 1));
            itemSvc.GetRange(MyCompanyFile, pageFilter, MyCredentials, OnComplete, OnError);
        }

        /// <summary>
        /// Method called on Async complete
        /// </summary>
        /// <param name="statusCode"></param>
        /// <param name="items"></param>
        /// <remarks></remarks>
        private void OnComplete(System.Net.HttpStatusCode statusCode,
                                PagedCollection<Item> items)
        {
            myItems = items;
        }

    /// <summary>
    /// Callback if there is an error
    /// </summary>
    /// <param name="uri"></param>
    /// <param name="ex"></param>
    /// <remarks></remarks>
    private void OnError(Uri uri, Exception ex)
    {
        Trace.WriteLine("In OnError");

        MessageBox.Show(ex.Message);
    }

I want code something like this

private async Task FetchItemsAsync()
{
        var itemSvc = new ItemService(MyConfiguration, null, MyOAuthKeyService);

        string pageFilter = string.Format("$top={0}&$skip={1}&$orderby=Date desc", PageSize,
                                          PageSize * (_currentPage - 1));

        itemSvc.GetRange(MyCompanyFile, pageFilter, MyCredentials, OnComplete, OnError);

        var totalPages = (int)Math.Ceiling((double)(myItems.Count  / PageSize));

        while (_currentPage < totalPages)
        {
            await LoadMore(); // how do I write this?
        }
}

How do I do that?

[Update5]

I tried

    private const double PageSize = 400;
    protected CancellationTokenSource MyCancellationTokenSource;
    protected CompanyFile MyCompanyFile;
    protected IApiConfiguration MyConfiguration;
    protected ICompanyFileCredentials MyCredentials;

    protected ItemService MyItemService;
    protected IOAuthKeyService MyOAuthKeyService;

    private int _currentPage = 1;
    private int _totalPages;

    public void FetchItems(CompanyFile companyFile, IApiConfiguration configuration, ICompanyFileCredentials credentials)
    {
        MyCompanyFile = companyFile;
        MyConfiguration = configuration;
        MyCredentials = credentials;
        MyCancellationTokenSource = new CancellationTokenSource();
        MyItemService = new ItemService(MyConfiguration, null, MyOAuthKeyService);
        FetchAllItemsAsync();
    }

    private async void FetchAllItemsAsync()
    {
        try
        {
            var items = new List<Item>();
            int totalPages = 0;
            do
            {
                string pageFilter = string.Format("$top={0}&$skip={1}&$orderby=Date desc", PageSize, PageSize * (_currentPage - 1));
                CancellationToken ct = MyCancellationTokenSource.Token;
                Log("About to Await GetRange");

                Task<PagedCollection<Item>> tpc = MyItemService.GetRangeAsync(MyCompanyFile, pageFilter, MyCredentials, ct, null);
                Log("About to Await GetRange B");

                PagedCollection<Item> newItems = await tpc;  // fails here

                Log("Page {0} retrieved {1} items", _currentPage, newItems.Count);
                if (totalPages == 0)
                {
                    totalPages = (int)Math.Ceiling((items.Count / PageSize));
                }
                items.AddRange(newItems.Items.ToArray());
                _currentPage++;
            }
            while (_currentPage < totalPages);
            MessageBox.Show(string.Format("Fetched {0} items", items.Count));
        }
        catch (ApiCommunicationException ex)
        {
            Log(ex.ToString());
            throw;
        }
        catch (Exception exception)
        {
            Log(exception.ToString());
            throw;
        }
    }

However I get a ValidationException

{"Encountered a validation error (http://localhost:8080/AccountRight/ab5c1f96-7663-4052-8360-81004cfe8598/Inventory/Item/?$top=400&$skip=0&$orderby=Date desc)"}
    [MYOB.AccountRight.SDK.ApiValidationException]: {"Encountered a validation error (http://localhost:8080/AccountRight/ab5c1f96-7663-4052-8360-81004cfe8598/Inventory/Item/?$top=400&$skip=0&$orderby=Date desc)"}
    base: {"Encountered a validation error (http://localhost:8080/AccountRight/ab5c1f96-7663-4052-8360-81004cfe8598/Inventory/Item/?$top=400&$skip=0&$orderby=Date desc)"}
    ErrorInformation: "Warning, error messages have not been finalised in this release and may change"
    Errors: Count = 1
    RequestId: "e573dfed-ec68-4aff-ac5e-3ffde1c2f943"
    StatusCode: BadRequest
    URI: {http://localhost:8080/AccountRight/ab5c1f96-7663-4052-8360-81004cfe8598/Inventory/Item/?$top=400&$skip=0&$orderby=Date desc}

I have cross posted this problem to MYOB Support

Answer 1

The latest version of the MYOB.AccountRight.API.SDK you are referencing already has overloads for supporting async/await on .NET4, .NET45 and PCL.

The Sample code was created as an example for someone using .NET 3.5 (hence no async/await). Another sample (windows phone) shows async/await in action using the SDK

[Update]

You are probably getting an OData related exception as the Item entity does not have a Date field for which you can filter by (see docs).

When you catch an ApiCommunicationException (of which ApiValidationException is a subclass) there is an Errors property that provides more detail.

There is also a RequestId (and some other properties) which are very useful should you need to talk to the support guys if you have issues talking to the cloud hosted API.

Answer 2

You can use a TaskCompletionSource object that you can resolve with the result or error callback. I'm not sure what the signature of the error callback is so that part probably wont work.

private Task<PagedCollection<Item>> FetchItemsAsync()
{
    var taskSource = new TaskCompletionSource<PagedCollection<Item>>();

    var itemSvc = new ItemService(MyConfiguration, null, MyOAuthKeyService);
    string pageFilter = string.Format("$top={0}&$skip={1}&$orderby=Date desc", PageSize,
                                              PageSize * (_currentPage - 1));
    itemSvc.GetRange(
        MyCompanyFile,
        pageFilter,
        MyCredentials,
        (statusCode, items) => taskSource.TrySetResult(items), 
        (error) => taskSource => taskSource.TrySetException(error) // Not sure if this is correct signature
        );
    return taskSource.Task;
}

You can then return the Task object that it creates which you can use for async things. I'm not really sure about what logic you are trying to implement because your question is not very detailed but you can use the method with the await command because it returns a Task object like the following.

private async void FetchAllItemsAsync() 
{
    int totalPages;
    do
    {
        items = await FetchItemsAsync()
        totalPages = (int)Math.Ceiling((double)(items.Count / PageSize));
        _currentPage++;
    } while (_currentPage < totalPages)

}