StorageFile Async usage in a NON Metro application

问题:

I am trying to create an instance of StorageFile in my class library...

var localFolder = ApplicationData.Current.LocalFolder;
StorageFile destinationFile = await localFolder.CreateFileAsync(destination, CreationCollisionOption.GenerateUniqueName);

VS11 is not building say: 'await' requires that the type 'Windows.Foundation.IAsyncOperation' have a suitable GetAwaiter method. Are you missing a using directive for 'System'?

obviously I am using .net 4.5 as target and I am referencing Windows Assemblies... not sure why this code work in MetroStyle but does not build in a class library... How can I create an instance of Storagefile in a class library??? at this stage it is not important if the file is created in an async way...

pls let me know your thoughts... Stelio

回答1:

What worked for me is to "manually" add the TargetPlatformVersion

<PropertyGroup>
  <TargetPlatformVersion>8.0</TargetPlatformVersion>
</PropertyGroup>

and in Item group add the following

<ItemGroup>
  <Reference Include="System.Runtime.WindowsRuntime" />
  <Reference Include="System.Runtime" />
  <Reference Include="Windows" />
</ItemGroup>

Then the project should compile normaly.

回答2:

I had the same issue.The problem was system namespace is missing in the namespace header.I just included system in the namespace and it worked. hope it helps.

回答3:

Looks like you are trying to use a type from the WinRT libraries because the StorageFile class documentation states it applies to Metro only and it is found in Windows.Storage.

This blog post goes through how to build it, but it appears to be a manual process. It also details the cause of the error:

Using the await keyword causes the compiler to look for a GetAwaiter method on this interface. Since IAsyncOperation does not define a GetAwaiter method, the compiler wants to look for an extension method.

Basically, it looks like you need to add a reference to: System.Runtime.WindowsRuntime.dll

---

Please take the time to read his blog post, but I will put the important part here for clarity.

---

Blog Content Below Unceremoniously Plagiarised

First, in Notepad, I created the following C# source code in EnumDevices.cs:

using System;
using System.Threading.Tasks;
using Windows.Devices.Enumeration;
using Windows.Foundation;

class App {
    static void Main() {
        EnumDevices().Wait();
    }

    private static async Task EnumDevices() {
        // To call DeviceInformation.FindAllAsync:
        // Reference Windows.Devices.Enumeration.winmd when building
        // Add the "using Windows.Devices.Enumeration;" directive (as shown above)
        foreach (DeviceInformation di in await DeviceInformation.FindAllAsync()) {
            Console.WriteLine(di.Name);
        }
    }
}

Second, I created a Build.bat file which I run from the Developer Command Prompt to build this code (This should be 1 line but I wrap it here for read ability):

csc EnumDevices.cs  
/r:c:\Windows\System32\WinMetadata\Windows.Devices.Enumeration.winmd  
/r:c:\Windows\System32\WinMetadata\Windows.Foundation.winmd 
/r:System.Runtime.WindowsRuntime.dll  
/r:System.Threading.Tasks.dll

Then, at the command prompt, I just run the EnumDevices.exe to see the output.

回答4:

It must be a long time since the post when I After adding references:
System.Runtime.WindowsRuntime.dll
System.Threading.Tasks.dll

and targeting windows 8 in project file:

  <PropertyGroup>
    <TargetPlatformVersion>8.0</TargetPlatformVersion>
  </PropertyGroup>

Mentioned above example can be compiled in VS.