Blazor async guidance (#18958)

Author: guardrex

aspnetcore/blazor/components/index.md

@@ -5,7 +5,7 @@ description: Learn how to create and use Razor components, including how to bind
 monikerRange: '>= aspnetcore-3.1'
 ms.author: riande
 ms.custom: mvc
-ms.date: 06/11/2020
+ms.date: 06/25/2020
 no-loc: [Blazor, "Identity", "Let's Encrypt", Razor, SignalR]
 uid: blazor/components/index
 ---
@@ -416,12 +416,25 @@ While capturing component references use a similar syntax to [capturing element
 > [!NOTE]
 > Do **not** use component references to mutate the state of child components. Instead, use normal declarative parameters to pass data to child components. Use of normal declarative parameters result in child components that rerender at the correct times automatically.
 
-## Invoke component methods externally to update state
+## Synchronization context
 
 Blazor uses a synchronization context (<xref:System.Threading.SynchronizationContext>) to enforce a single logical thread of execution. A component's [lifecycle methods](xref:blazor/components/lifecycle) and any event callbacks that are raised by Blazor are executed on the synchronization context.
 
 Blazor Server's synchronization context attempts to emulate a single-threaded environment so that it closely matches the WebAssembly model in the browser, which is single threaded. At any given point in time, work is performed on exactly one thread, giving the impression of a single logical thread. No two operations execute concurrently.
 
+### Avoid thread-blocking calls
+
+Generally, don't call the following methods. The following methods block the thread and thus block the app from resuming work until the underlying <xref:System.Threading.Tasks.Task> is complete:
+
+* <xref:System.Threading.Tasks.Task%601.Result%2A>
+* <xref:System.Threading.Tasks.Task.Wait%2A>
+* <xref:System.Threading.Tasks.Task.WaitAny%2A>
+* <xref:System.Threading.Tasks.Task.WaitAll%2A>
+* <xref:System.Threading.Thread.Sleep%2A>
+* <xref:System.Runtime.CompilerServices.TaskAwaiter.GetResult%2A>
+
+### Invoke component methods externally to update state
+
 In the event a component must be updated based on an external event, such as a timer or other notifications, use the `InvokeAsync` method, which dispatches to Blazor's synchronization context. For example, consider a *notifier service* that can notify any listening component of the updated state:
 
 ```csharp