Using TPL to avoid callback hell

In this example, we fire off three service calls which individually have a continuation of converting the resultant DTO to a model object.

We then call a fourth service, to get the Spot which only fires once all the previous three service calls have completed and created their model object.

var tasks = new Task[]
{
    services.GetAsTask<DTOs.MoneyMarketRate>()
            .ContinueWith(t => MoneyMarketRate = Mapper.Map<DTOs.MoneyMarketRate, MoneyMarketRate>(t.Result)),

    services.GetAsTask<DTOs.InvestmentBoundaries>()
            .ContinueWith(t => InvestmentBoundaries = Mapper.Map<DTOs.InvestmentBoundaries, InvestmentBoundaries>(t.Result)),

    services.GetAsTask<DTOs.TradingDate>()
            .ContinueWith(t => TradingDate = Mapper.Map<DTOs.TradingDate, TradingDate>(t.Result)),
};

Task.Factory.ContinueWhenAll(
    tasks,
    ts => services.GetAsTask<DTOs.Spot>()
                    .ContinueWith(t => Spot = Mapper.Map<DTOs.Spot, Spot>(t.Result)));

You can take a look at my Silverlight source project here:  https://github.com/stevenh77/UsingTasksToAvoidCallbackHell
The problem with this code is that the exceptions are not properly handled.  For that we can leverage async and await:

try 
{
    var tasks = new []
    {
        services.GetAsync<DTOs.MoneyMarketRate, MoneyMarketRate>(x => MoneyMarketRate = x),
        services.GetAsync<DTOs.InvestmentBoundaries, InvestmentBoundaries>(x => InvestmentBoundaries = x),
        services.GetAsync<DTOs.TradingDate, TradingDate>(x => TradingDate = x),
    };

    await TaskEx.WhenAll(tasks);
    await services.GetAsync<DTOs.Spot, Spot>(x => Spot = x);
}
catch (Exception e)
{
    MessageBox.Show(e.Message);
}

I also refactored the service executor to use async and await and move the setting to the property to an action that can be passed in.

public class ServiceExecutor
{
    const string BaseAddress = "http://localhost:8080/Services/";

    public async Task GetAsync<TDto, TModel>(Action<TModel> actionToSetPropertyValue)
    {
        var client = new WebClient();
        var serviceName = typeof(TDto).Name + ".ashx";
        var response = await client.DownloadStringTaskAsync(new Uri(BaseAddress + serviceName, UriKind.Absolute));
        var dto = JsonConvert.DeserializeObject<TDto>(response);
        actionToSetPropertyValue.Invoke(Mapper.Map<TDto, TModel>(dto));
    }
}

Refactored source:  https://github.com/stevenh77/UsingTasksToAvoidCallbackHellWithAsyncAndWait/