如何在 AsAsyncOperation 中进行强制转换？

Question

我正在参加有关 WinRT IAsyncOperation、异步、等待的速成课程。我正在尝试使用 IAsyncOperation 包装 Task，但未能想出一个简单的方法来进行转换。

public interface IInfo{}
public class InfoList : IInfo{}

public class Parser {
    private async Task<InfoList> ParseList() {
        var returnList = new InfoList();

        HttpClient client = new HttpClient();
        Task<string> htmlTask = client.GetStringAsync("http://stackoverflow.com");

        string html = await htmlTask;
        // parsing logic...
        return returnList;
    }

    public IAsyncOperation<IInfo> Parse()
    {
        return AsyncInfo.Run(_ =>
            Task.Run<IInfo>(async () =>
            {
                var returnList = await ParseList();
                return returnList;
            })
        );
        // I have tried following but causes InvalidCastException
        //return (IAsyncOperation<IInfo>)ParseList().AsAsyncOperation<InfoList>();
    }
}

Answer 1

public IAsyncOperation<IInfo> Parse()
{
    return AsyncInfo.Run(async _ => (IInfo) await ParseList());
}

或

public IAsyncOperation<IInfo> Parse()
{
    return AsyncInfo.Run(_ => ParseList().ContinueWith(t => (IInfo)t.Result));
}

Answer 2

IAsyncOperation<> 类型不是协变的，这意味着您不能将 IAsyncOperation<InfoList> 强制转换为 IAsyncOperation<IInfo>。

尝试让您的ParseList 方法返回Task<IInfo>。