Parallel.ForEach不适用于async动作，但是您可以启动所有任务，然后使用Task.WhenAll等待它们全部完成:

Parallel.ForEach doesn't work with async actions, but you could start all tasks and then wait for them all to complete using Task.WhenAll:

[HttpPost]
public async Task<IHttpActionResult> GetAccountsAsync(IEnumerable<int> accountIds)
{
    Task<List<AccountDetail>>[] tasks = accountIds.Select(accountId => GetAccountDetailAsync(accountId)).ToArray();
    List<AccountDetail>[] results = await Task.WhenAll(tasks);
    return Ok(results.SelectMany(x => x).ToList());
}