ConfigureAwaitとSynchronizationContext

SynchronizationContextとは

ConfigureAwait の仕組み

// Default: captures context (resumes on original context)
var data = await FetchDataAsync();
UpdateUI(data); // runs on UI thread — correct for UI apps

// ConfigureAwait(false): doesn't capture context
var data2 = await FetchDataAsync().ConfigureAwait(false);
// Runs on any thread — DON'T access UI thread-only objects here

典型的なデッドロックパターン

// DEADLOCK in WPF/WinForms or ASP.NET classic:
string result = GetDataAsync().Result; // blocks UI thread
// GetDataAsync() tries to resume on UI thread after await
// UI thread is blocked by .Result
// DEADLOCK!

// Fix 1: await properly (never block)
string result = await GetDataAsync();

// Fix 2: use ConfigureAwait(false) in the library
public async Task<string> GetDataAsync()
    => await client.GetStringAsync(url).ConfigureAwait(false);

ライブラリでの ConfigureAwait(false)

// Library code — always ConfigureAwait(false)
public async Task<User?> GetUserAsync(int id)
{
    var json = await client
        .GetStringAsync($"/users/{id}")
        .ConfigureAwait(false);

    var user = JsonSerializer.Deserialize<User>(json);
    return user;
}

// Application code (has context) — leave ConfigureAwait default
public async Task LoadUserProfileAsync(int id)
{
    var user = await userService.GetUserAsync(id); // context captured
    ProfileLabel.Text = user?.Name; // safe to update UI
}

ASP.NET Core: SynchronizationContext なし

// In ASP.NET Core, both are equivalent:
var data = await service.GetAsync();                      // fine
var data2 = await service.GetAsync().ConfigureAwait(false); // also fine

// Context-free means: no deadlock risk from .Result in ASP.NET Core
// BUT: mixing with libraries that assume a context is still risky
// BEST PRACTICE: still use ConfigureAwait(false) in library code

スレッドプールとコンテキストによるスケジューリング

// Check current synchronization context:
Console.WriteLine(SynchronizationContext.Current?.GetType().Name
    ?? "No context (thread pool)");

// In a WPF event handler: "DispatcherSynchronizationContext"
// In ASP.NET Core: null (no context)
// In a unit test with xUnit: "AsyncTestSyncContext"

// After ConfigureAwait(false):
await Task.Delay(1).ConfigureAwait(false);
Console.WriteLine(SynchronizationContext.Current?.GetType().Name
    ?? "No context"); // null — context was abandoned

ループ内での ConfigureAwait

// Library loop: ConfigureAwait(false) on every await
public async Task ProcessBatchAsync(IEnumerable<int> ids)
{
    foreach (var id in ids)
    {
        var item = await FetchItemAsync(id).ConfigureAwait(false);
        await SaveItemAsync(item).ConfigureAwait(false);
    }
}

ValueTask と ConfigureAwait

public async ValueTask<string> GetFromCacheAsync(string key)
{
    // ValueTask with ConfigureAwait(false)
    var raw = await _store.GetAsync(key).ConfigureAwait(false);
    return raw ?? "";
}

コンテキストを回避する Task.Run

// WRONG in UI app: blocks UI thread
void ButtonClick(object? sender, EventArgs e)
{
    var result = HeavyComputation(); // blocks UI
    Label.Text = result;
}

// CORRECT: offload to thread pool
async void ButtonClick(object? sender, EventArgs e)
{
    var result = await Task.Run(() => HeavyComputation());
    // Continuation captured UI context: safe to update UI
    Label.Text = result;
}

実践例: ConfigureAwait を使用する HttpClient ライブラリ

public class ApiClient
{
    private readonly HttpClient _http;
    private readonly JsonSerializerOptions _opts = new(JsonSerializerDefaults.Web);

    public ApiClient(HttpClient http) => _http = http;

    public async Task<T> GetAsync<T>(string path, CancellationToken ct = default)
    {
        using var response = await _http
            .GetAsync(path, ct)
            .ConfigureAwait(false);

        response.EnsureSuccessStatusCode();

        return await response.Content
            .ReadFromJsonAsync<T>(_opts, ct)
            .ConfigureAwait(false)
            ?? throw new InvalidOperationException("Null response");
    }
}

確認問題

まとめ: ConfigureAwait と SynchronizationContext