The await operator can only be used within an async method

Question

I have an interface ISFactory as follows.

namespace MyApp.ViewModels
{
    public interface IStreamFactory
    {
        Stream CreateSPStream(string sPName);
    }
}

On Windows non-universal version the above function was implemented as follows.

public Stream CreateSerialPortStream(string serialPortName)
{
    var p = new System.IO.Ports.SerialPort();
    p.PortName = serialPortName;
    p.BaudRate = 9600;
    p.RtsEnable = true;
    p.DtrEnable = true;
    p.ReadTimeout = 150;
    p.Open();
    return p.BaseStream;
}

This implementation is no longer available in Windows Universal. What I attempted is shown below.

public  Stream CreateSerialPortStream(string serialPortName)
{
    var selector = SerialDevice.GetDeviceSelector(serialPortName); //Get the serial port on port '3'
    var devices = await DeviceInformation.FindAllAsync(selector);
    if (devices.Any()) //if the device is found
    {
        var deviceInfo = devices.First();
        var serialDevice = await SerialDevice.FromIdAsync(deviceInfo.Id);
        //Set up serial device according to device specifications:
        //This might differ from device to device
        serialDevice.BaudRate = 19600;
        serialDevice.DataBits = 8;
        serialDevice.Parity = SerialParity.None;
    }
}

I get the following error.

The await operator can only be used within an async method.`

Can anyone suggest a way around this.

`public async Task CreateSerialPortStream(string serialPortName)` — Nkosi, Sep 11 '16 at 22:16
@Nkosi - Can I get `Stream` from `Task`. I am passing `stream` to other functions which is failing now. — liv2hak, Sep 11 '16 at 22:20
I may have spoken too soon. check `serialDevice.InputStream` — Nkosi, Sep 11 '16 at 22:24
@Nkosi - But still I have to change the return type to Task ? — liv2hak, Sep 11 '16 at 22:25
Yes if you are doing `async/await` then you have to return a `Task` — Nkosi, Sep 11 '16 at 22:25
https://msdn.microsoft.com/en-us/library/windows/apps/windows.devices.serialcommunication.serialdevice.inputstream.aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-1 — Nkosi, Sep 11 '16 at 22:30
Take a look at this SO question http://stackoverflow.com/questions/34160376/windows-universal-app-serial-ports-wont-open-serialdevice-fromidasync-always-n — Nkosi, Sep 11 '16 at 22:32

score 10 · Answer 1 · answered Sep 11 '16 at 22:25

10

The best approach is to make the method async, as the compiler error indicates:

public async Task<Stream> CreateSerialPortStreamAsync(string serialPortName)

This will require the interface to change as well:

Task<Stream> CreateSerialPortStreamAsync(string serialPortName);

And yes, all callers of this method will need to be updated.

answered Sep 11 '16 at 22:25

Stephen Cleary

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What will I return from the function? – liv2hak Sep 11 '16 at 22:29
1

@liv2hak you just return a `Stream`, which is automatically wrapped in a `Task` – zerkms Sep 11 '16 at 23:05
@zerkms - When I add async to the interface function, it says modified async is not valid for this item. – liv2hak Sep 11 '16 at 23:12
1

@liv2hak `async` keyword is a part of the implementation, not the interface. Just go through the answer once again - it mentions that the signature of the method and of the interface change slightly differently. – zerkms Sep 11 '16 at 23:18

score 2 · Answer 2 · answered Sep 11 '16 at 23:03

1) (Preferable) You can make your method async by changing its sugnature to :

public  async Task<Stream> CreateSerialPortStream(string serialPortName)

each method calling CreateSerialPortStream should be async too. (Also i suggest to rename your method to CreateSerialPortStreamAsync)

2) If you don't want to change your method signature for some reason, you can leave it as is, but use Wait(). In this case, you will lose asynchronous calling.

public  Stream CreateSerialPortStream(string serialPortName)
{
    var selector = SerialDevice.GetDeviceSelector(serialPortName); //Get the serial port on port '3'
    var devicesTask = await DeviceInformation.FindAllAsync(selector);
    devicesTask.Wait();
    var devices = devicesTask.Result;
    if (devices.Any()) //if the device is found
    {
            var deviceInfo = devices.First();
            var serialDeviceTask = SerialDevice.FromIdAsync(deviceInfo.Id);
            serialDeviceTask.Wait();
            var serialDevice = serialDeviceTask.Result;        
            //Set up serial device according to device specifications:
            //This might differ from device to device
            serialDevice.BaudRate = 19600;
            serialDevice.DataBits = 8;
            serialDevice.Parity = SerialParity.None;
      }
}

The await operator can only be used within an async method

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