Task Extensions

Task Extension.cs

static async Task<T> Otherwise<T> (this Task<T> task, Func<Task<T>> orTask) {
    task.ContinueWith (async innerTask => {
        if (innerTask.Status == TaskStatus.Faulted)
            return await orTask ();

        return await Task.FromResult<T> (innerTask.Result);
    }).Unwrap ();
}

static async Task<T> Retry<T> (Func<Task<T>> task, int retries, TimeSpan delay, CancellationToken cts = default (CancellationToken)) {
    await task ().ContinueWith (async innerTask => {
        cts.ThrowIfCancellationRequested ();
        if (innerTask.Status != TaskStatus.Faulted)
            return innerTask.Result;
        if (retries == 0)
            throw innerTask.Exception ??
                throw new Exception ();

        await Task.Delay (delay, cts);
        return await Retry (task, retries - 1, delay, cts);
    }).Unwrap ();
}

// Example: 
Image image = await AsyncEx.Retry (async () =>
    await DownloadImageAsync ("Bugghina001.jpg").Otherwise (async () =>await DownloadImageAsync ("Bugghina002.jpg")),
    5, TimeSpan.FromSeconds (2));
    
static Task<T> Catch<T, TError> (this Task<T> task, Func<TError, T> onError) where TError : Exception {
    var tcs = new TaskCompletionSource<T> ();
    task.ContinueWith (innerTask => {
        if (innerTask.IsFaulted && innerTask?.Exception?.InnerException is TError)
            tcs.SetResult (onError ((TError) innerTask.Exception.InnerException));
        else if (innerTask.IsCanceled)
            tcs.SetCanceled ();
        else if (innerTask.IsFaulted)
            tcs.SetException (innerTask?.Exception?.InnerException ??throw new InvalidOperationException ());
        else
            tcs.SetResult (innerTask.Result);
    });
    return tcs.Task;
}