ConfigureAwaitと例外の伝播

コンテキストと例外

ライブラリのベストプラクティス

using System;
using System.Threading.Tasks;

public static class NetLib
{
  // Library code should not assume a UI context. Use ConfigureAwait(false).
  public static async Task<string> FetchAsync()
  {
    await Task.Delay(100).ConfigureAwait(false); // do not capture context
    return "payload";
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    // Console apps typically have no SynchronizationContext; this still runs fine.
    string s = NetLib.FetchAsync().GetAwaiter().GetResult();
    Console.WriteLine("Got: " + s);
  }
}

コンテキストのルール

Await と例外

using System;
using System.Threading.Tasks;

public class Program
{
  static async Task<string> BoomAsync()
  {
    await Task.Delay(50);
    throw new InvalidOperationException("boom");
  }

  public static void Main(string[] args)
  {
    // With await (inside async), exceptions rethrow as the original type.
    try
    {
      string r = BoomAsync().GetAwaiter().GetResult(); // emulate await in Main
      Console.WriteLine(r);
    }
    catch (InvalidOperationException ex)
    {
      Console.WriteLine("Caught: " + ex.GetType().Name + " - " + ex.Message);
    }
  }
}

ブロッキングとエラー

using System;
using System.Threading.Tasks;

public class Program
{
  static async Task<int> FailsAsync()
  {
    await Task.Delay(30);
    throw new ArgumentException("bad arg");
  }

  public static void Main(string[] args)
  {
    try
    {
      // Blocking with .Result wraps the exception into AggregateException
      int x = FailsAsync().Result;
      Console.WriteLine(x);
    }
    catch (AggregateException ae)
    {
      Exception inner = ae.InnerException;
      Console.WriteLine("AggregateException -> " + (inner == null ? "null" : inner.GetType().Name));
    }
  }
}

ベストプラクティス

ConfigureAwait の動作

まとめ