Why Does My Asynchronous C# Method Hang When Accessing Task Results?

The reason why C# asynchronous method hangs when accessing Task results

When using C#'s async and await keywords for asynchronous programming, certain constructs can cause unexpected behavior and potential deadlocks.

Consider the following scenario: A multi-tier application uses an extended database utility method ExecuteAsync that asynchronously executes a SQL query and returns the results. The middle layer method GetTotalAsync calls ExecuteAsync to retrieve the data and stores the result in the asyncTask variable. Finally, UI operations attempt to access the results synchronously using asyncTask.Result. However, the application hangs indefinitely.

Cause of deadlock

The problem arises from using GetTotalAsync in the await method. By default, continuations of async methods are dispatched on the same SynchronizationContext that started the method. In this case, when using await on the UI thread, the continuation (return result;) is also scheduled to run on the UI thread.

When asyncTask.Result is called on the UI thread, it blocks the thread when the Task completes. However, continuations scheduled on the UI thread cannot execute until asyncTask.Result completes. This creates a deadlock where neither thread can continue execution.

Solution

In order to solve this deadlock, there are several methods:

1. Delete Async keyword:

Eliminate the use of await and rewrite the ExecuteAsync and GetTotalAsync methods as pure asynchronous methods that do not wait:

<code class="language-csharp">public static Task<T> ExecuteAsync<T>(this OurDBConn dataSource, Func<OurDBConn, T> function)
{
    // ... (代码保持不变)
}

public static Task<ResultClass> GetTotalAsync(...)
{
    // ... (代码保持不变)
}</code>

2. Use ConfigureAwait:

Use ConfigureAwait(false) to specify that continuations should not be scheduled on the UI thread:

<code class="language-csharp">public static async Task<ResultClass> GetTotalAsync(...)
{
    var resultTask = this.DBConnection.ExecuteAsync<ResultClass>(
        ds => ds.Execute("select slow running data into result"));

    return await resultTask.ConfigureAwait(false);
}</code>

Note that this approach requires explicit specification of await on all ConfigureAwait(false) operations that may lead to deadlock.

3. Use SynchronizationContext:

Create a specific SynchronizationContext for asynchronous operations and ensure that all await operations use that context, preventing conflicts with the UI thread.

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