How to: Use AsyncPackage to Load VSPackages in the Background

Loading and initializing a VS package can result in disk I/O. If such I/O happens on the UI thread, it can lead to responsiveness issues. To address this, Visual Studio 2015 introduced the AsyncPackage class that enables package loading on a background thread.

Creating an AsyncPackage

You can start by creating a VSIX project (File / New / Project / Visual C# / Extensibility / VSIX Project) and adding a VSPackage to the project (right click on the project and Add/New Item/C# item/Extensibility/Visual Studio Package). You can then create your services and add those services to your package.

  1. Derive the package from AsyncPackage.

  2. If you are providing services whose querying may cause your package to load:

    To indicate to Visual Studio that your package is safe for background loading and to opt into this behavior, your PackageRegistrationAttribute should set AllowsBackgroundLoading property to true in the attribute constructor.

    [PackageRegistration(UseManagedResourcesOnly = true, AllowsBackgroundLoading = true)]  

    To indicate to Visual Studio that it is safe to instantiate your service on a background thread, you should set the IsAsyncQueryable property to true in the ProvideServiceAttribute constructor.

    [ProvideService(typeof(SMyTestService), IsAsyncQueryable = true)]  
  3. If you are loading via UI contexts, then you should specify PackageAutoLoadFlags.BackgroundLoad for the ProvideAutoLoadAttribute OR the value (0x2) into the flags written as the value of your package’s auto-load entry.

    [ProvideAutoLoad(UIContextGuid, PackageAutoLoadFlags.BackgroundLoad)]  
  4. If you have asynchronous initialization work to do, you should override InitializeAsync. Remove the Initialize() method provided by the VSIX template. (The Initialize() method in AsyncPackage is sealed). You can use any of the <xref:Microsoft.VisualStudio.Shell.AsyncPackage.AddService%2A> methods to add asynchronous services to your package.

    NOTE: To call base.InitializeAsync(), you can change your source code to:

    await base.InitializeAsync(cancellationToken, progress);  
  5. You must take care to NOT make RPCs (Remove Procedure Call) from your asynchronous initialization code (in InitializeAsync). These can occur when you call GetService directly or indirectly. When sync loads are required, the UI thread will block using JoinableTaskFactory. The default blocking model disables RPCs. This means that if you attempt to use an RPC from your async tasks, you will deadlock if the UI thread is itself waiting for your package to load. The general alternative is to marshal your code to the UI thread if needed using something like Joinable Task Factory's SwitchToMainThreadAsync or some other mechanism that does not use an RPC. Do NOT use ThreadHelper.Generic.Invoke or generally block the calling thread waiting to get to the UI thread.

    NOTE: You should avoid using GetService or QueryService in your InitializeAsync method. If you have to use those, you will need to switch to the UI thread first. The alternative is to use <xref:Microsoft.VisualStudio.Shell.AsyncServiceProvider.GetServiceAsync%2A> from your AsyncPackage (by casting it to IAsyncServiceProvider.)

    C#: Create an AsyncPackage :

[PackageRegistration(UseManagedResourcesOnly = true, AllowsBackgroundLoading = true)]       
[ProvideService(typeof(SMyTestService), IsAsyncQueryable = true)]   
public sealed class TestPackage : AsyncPackage   
    protected override Task InitializeAsync(System.Threading.CancellationToken cancellationToken, IProgress<ServiceProgressData> progress)   
        this.AddService(typeof(SMyTestService), CreateService, true);   
        return Task.FromResult<object>(null);   

Convert an existing VSPackage to AsyncPackage

The majority of the work is the same as creating a new AsyncPackage. You need to follow steps 1 through 5 above. You also need to take extra caution on the following:

  1. Remember to remove the Initialize override you had in your package.

  2. Avoid deadlocks: There could be hidden RPCs in your code which now happen on a background thread. You need to ensure that if you are making an RPC (e.g. GetService), you need to either (1) switch to the main thread or (2) use the asynchronous version of the API if one exists (e.g. GetServiceAsync).

  3. Do not switch between threads too frequently. Try to localize the work that can happen in a background thread. This reduces the load time.

Querying Services from AsyncPackage

An AsyncPackage may or may not load asynchronously depending on the caller. For instance,

  • If the caller called GetService or QueryService (both synchronous APIs) or

  • If the caller called IVsShell::LoadPackage (or IVsShell5::LoadPackageWithContext) or

  • The load is triggered by a UI context, but you did not specify the UI context mechanism can load you asynchronously

    then your package will load synchronously.

    Note that your package still has an opportunity (in its asynchronous initialization phase) to do work off the UI thread, though the UI thread will be blocked for that work’s completion. If the caller uses IAsyncServiceProvider to asynchronously query for your service, then your load and initialization will be done asynchronously assuming they don’t immediately block on the resulting task object.

    C#: How to query service asynchronously:

using Microsoft.VisualStudio.Shell;   
using Microsoft.VisualStudio.Shell.Interop;   

IAsyncServiceProvider asyncServiceProvider = Package.GetService(typeof(SAsyncServiceProvider)) as IAsyncServiceProvider;   
IMyTestService testService = await ayncServiceProvider.GetServiceAsync(typeof(SMyTestService)) as IMyTestService;