Calling Async in a Sync method

Question

I've been reading examples for a long time now, but unfortunately I've been unable to apply the solutions to the code I'm working with. Some quick Facts/Assorted Info:

1) I'm new to C#

2) The code posted below is modified from Amazon Web Services (mostly stock)

3) Purpose of code is to compare server info to offline already downloaded info and create a list of need to download files. This snip is for the list made from the server side, only option with AWS is to call async, but I need this to finish before moving forward.

public void InitiateSearch()
{
    UnityInitializer.AttachToGameObject(this.gameObject);
    //these are the access key and secret access key for credentials
    BasicAWSCredentials credentials = new BasicAWSCredentials("secret key", "very secret key");
    AmazonS3Config S3Config = new AmazonS3Config()
    {
        ServiceURL = ("url"),
        RegionEndpoint = RegionEndpoint.blahblah

    };
    //Setting the client to be used in the call below
    AmazonS3Client Client = new AmazonS3Client(credentials, S3Config);
    var request = new ListObjectsRequest()
    {
        BucketName = "thebucket"
    };

    Client.ListObjectsAsync(request, (responseObject) =>
    {

        if (responseObject.Exception == null)
        {
            responseObject.Response.S3Objects.ForEach((o) =>
            {
                int StartCut = o.Key.IndexOf(SearchType) - 11;
                if (SearchType == o.Key.Substring(o.Key.IndexOf(SearchType), SearchType.Length))
                {
                    if (ZipCode == o.Key.Substring(StartCut + 12 + SearchType.Length, 5))
                    {
                        AWSFileList.Add(o.Key + ", " + o.LastModified);
                    }
                }
            }

            );
        }
        else
        {
            Debug.Log(responseObject.Exception);
        }
    });
}

I have no idea how to apply await to the Client.ListObjectsAsync line, I'm hoping you all can give me some guidance and let me keep my hair for a few more years.

score 1 · Answer 1 · answered Sep 03 '16 at 03:10

1

You can either mark your method async and await it, or you can call .Wait() or .Result() on the Task you're given back.

answered Sep 03 '16 at 03:10

Collin Stevens

737
9
19

So in this situation AWSFileList is getting populated then it goes back to the main method that calls all the others. Where should I apply the .wait() or .result()? I've tried marking this method asysnc but it throws an error each time. – Icewing726 Sep 03 '16 at 04:02

score 1 · Answer 2 · answered Sep 03 '16 at 23:09

I have no idea how to apply await to the Client.ListObjectsAsync line

You probably just put await in front of it:

await Client.ListObjectsAsync(request, (responseObject) => ...

As soon as you do this, Visual Studio will give you an error. Take a good look at the error message, because it tells you exactly what to do next (mark InitiateSearch with async and change its return type to Task):

public async Task InitiateSearchAsync()

(it's also a good idea to add an Async suffix to follow the common pattern).

Next, you'd add an await everywhere that InitiateSearchAsync is called, and so on.

score 0 · Answer 3 · answered Sep 03 '16 at 19:16

I'm assuming Client.ListObjectsAsync returns a Task object, so a solution for your specific problem would be this:

public async void InitiateSearch()
{
    //code
    var collection = await Client.ListObjectsAsync(request, (responseObject) =>
    {
        //code
    });
    foreach (var item in collection)
    {
        //do stuff with item
    }
}

the variable result will now be filled with the objects. You may want to set the return type of InitiateSearch() to Task, so you can await it too.

await InitiateSearch(); //like this

If this method is an event handler of some sort (like called by the click of a button), then you can keep using void as return type.

A simple introduction from an unpublished part of the documentation for async-await:

Three things are needed to use async-await:

The Task object: This object is returned by a method which is executed asynchronous. It allows you to control the execution of the method.
The await keyword: "Awaits" a Task. Put this keyword before the Task to asynchronously wait for it to finish
The async keyword: All methods which use the await keyword have to be marked as async

A small example which demonstrates the usage of this keywords

public async Task DoStuffAsync()
{
    var result = await DownloadFromWebpageAsync(); //calls method and waits till execution finished
    var task = WriteTextAsync(@"temp.txt", result); //starts saving the string to a file, continues execution right await
    Debug.Write("this is executed parallel with WriteTextAsync!"); //executed parallel with WriteTextAsync!
    await task; //wait for WriteTextAsync to finish execution
}

private async Task<string> DownloadFromWebpageAsync()
{
    using (var client = new WebClient())
    {
        return await client.DownloadStringTaskAsync(new Uri("http://stackoverflow.com"));
    }
}

private async Task WriteTextAsync(string filePath, string text)
{
    byte[] encodedText = Encoding.Unicode.GetBytes(text);

    using (FileStream sourceStream = new FileStream(filePath, FileMode.Append))
    {
        await sourceStream.WriteAsync(encodedText, 0, encodedText.Length);
    }
}

Some thing to note:

You can specify a return value from an asynchronous operations with Task. The await keyword waits till the execution of the method finishes, and returns the string.
the Task object contains the status of the execution of the method, it can be used as any other variable.
if an exception is thrown (for example by the WebClient) it bubbles up at the first time the await keyword is used (in this example at the line string result (...))
It is recommended to name methods which return the Task object as MethodNameAsync

For more information about this take a look at http://blog.stephencleary.com/2012/02/async-and-await.html.

Calling Async in a Sync method

3 Answers3