无法从标记为async的方法返回IObservable <T>

这个人为的例子大致是我的代码的结构:

public abstract class SuperHeroBase
{
    protected SuperHeroBase() { }

    public async Task FightCrimeAsync()
    {
        var result = new CrimeFightingResult();

        result.State = CrimeFightingStates.Fighting;

        try
        {
            await FightCrimeOverride(results);
        } 
        catch
        {
            SetError(results);
        }

        if (result.State == CrimeFightingStates.Fighting)
            result.State = CrimeFightingStates.GoodGuyWon;

        return result;
    }

    protected SetError(CrimeFightingResult results)
    {
        result.State = CrimeFightingStates.BadGuyWon;
    }

    protected abstract Task FightCrimeOverride(CrimeFightingResult results);
}

public enum CrimeFightingStates
{
    NotStarted,
    Fighting,
    GoodGuyWon, // success state
    BadGuyWon // error state
}

public class CrimeFightingResult
{
    internal class CrimeFightingResult() { }

    public CrimeFightingStates State { get; internal set; }
}

现在我正在尝试构建一个可以容纳多个SuperHero对象并提供AllHerosFightCrime方法的集合.英雄不应该一次全部战斗(下一个在第一个完成时开始).

public class SuperHeroCollection : ObservableCollection
{
    public SuperHeroCollection() { }

    // I mark the method async...
    public async IObservable AllHerosFightCrime()
    {
        var heros = new List(this);

        var results = new ReplaySubject();

        foreach (var hero in heros)
        {
            // ... so I can await on FightCrimeAsync and push 
            // the result to the subject when done
            var result = await hero.FightCrimeAsync();
            results.OnNext(result);
        }

        results.OnCompleted();

        // I can't return the IObservable here because the method is marked Async.
        // It expects a return type of CrimeFightingResult
        return results;
    }
}

我该如何归还IObservable仍然有FightCrimeAsync等待的电话？