Managing Multiple Threads in Managed Code

Note

This article applies to Visual Studio 2015. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here

If you have a managed VSPackage extension that calls asynchronous methods or has operations that execute on threads other than the Visual Studio UI thread, you should follow the guidelines given below. You can keep the UI thread responsive because it doesn’t need to wait for work on another thread to complete. You can make your code more efficient, because you don’t have extra threads that take up stack space, and you can make it more reliable and easier to debug because you avoid deadlocks and unresponsiveness.

In general, you can switch from the UI thread to a different thread, or vice versa. When the method returns, the current thread is the thread from which it was originally called.

Important

The following guidelines use the APIs in the Microsoft.VisualStudio.Threading namespace, in particular, the JoinableTaskFactory class. The APIs in this namespace are new in Visual Studio 2013. You can get an instance of a JoinableTaskFactory from the ThreadHelper property ThreadHelper.JoinableTaskFactory.

Switching from the UI Thread to a Background Thread

1. If you are on the UI thread and you want to do asynchronous work on a background thread, use Task.Run():

   await Task.Run(async delegate{  
       // Now you’re on a separate thread.  
   });  
   // Now you’re back on the UI thread.  
   
   

2. If you are on the UI thread and you want to synchronously block while you are performing work on a background thread, use the TaskScheduler property TaskScheduler.Default inside Run:

   // using Microsoft.VisualStudio.Threading;  
   ThreadHelper.JoinableTaskFactory.Run(async delegate {  
       await TaskScheduler.Default;  
       // You're now on a separate thread.  
       DoSomethingSynchronous();  
       await OrSomethingAsynchronous();  
   });  
   

Switching from a Background Thread to the UI Thread

1. If you’re on a background thread and you want to do something on the UI thread, use SwitchToMainThreadAsync:

   // Switch to main thread  
   await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();  
   

   You can use the SwitchToMainThreadAsync method to switch to the UI thread. This method posts a message to the UI thread with the continuation of the current asynchronous method, and also communicates with the rest of the threading framework to set the correct priority and avoid deadlocks.

   If your background thread method isn’t asynchronous and you can’t make it asynchronous, you can still use the await syntax to switch to the UI thread by wrapping your work with Run, as in this example:

   ThreadHelper.JoinableTaskFactory.Run(async delegate {  
       // Switch to main thread  
       await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();  
       // Do your work on the main thread here.  
   });