不是所有的代码路径都返回Task上的值

Question

我需要等待整个任务完成然后返回一个值。我的代码给了我一个错误的"不是所有的代码路径返回一个值"。你能告诉我怎么修这个吗?我真的很困惑的任务。

public async override Task<TagInfoReply> GetInfo(TagInfoRequest request, ServerCallContext context)
{
var tagInfo = JsonSerializer.Deserialize<TagInfo>(request.TagInfo);
var parameter = JsonSerializer.Deserialize<ParameterGetWhiteListWithEntryInfo>(request.DetectedTag);
var task = Task.Run(() =>
{
return filters.FilterTags(tagInfo);
});
await task.ContinueWith(
async antecedent =>
{
if (antecedent.Result)
{
var reply = await pcs.GetTagInfo(parameter, tagInfo, TagFilterTime);
return new TagInfoReply
{
FetchedTagInfo = reply
};
}
else
{
return new TagInfoReply
{
FetchedTagInfo = string.Empty
};
}
}, TaskContinuationOptions.OnlyOnRanToCompletion);
}

如果我输入

return new TagInfoReply
{
FetchedTagInfo = string.Empty
};

在任务的最后和外部。ContinueWith，错误消失了，但问题是，当它运行时，它不会等待任务完成。它直接到最后一个返回。

Answer 1

目前，您的return代码仅从您传递给ContinueWith的匿名方法返回，并且它们将返回给ContinueWith内的调用代码。你实际上没有从GetInfo返回任何东西。

ContinueWith的结果作为其结果提供给您，因此您应该能够像这样重写代码:

public async override Task<TagInfoReply> GetInfo(TagInfoRequest request, ServerCallContext context)
{
var tagInfo = JsonSerializer.Deserialize<TagInfo>(request.TagInfo);
var parameter = JsonSerializer.Deserialize<ParameterGetWhiteListWithEntryInfo>(request.DetectedTag);
var task = Task.Run(() =>
{
return filters.FilterTags(tagInfo);
});
return await task.ContinueWith(
async antecedent =>
{
if (antecedent.Result)
{
var reply = await pcs.GetTagInfo(parameter, tagInfo, TagFilterTime);
return new TagInfoReply
{
FetchedTagInfo = reply
};
}
else
{
return new TagInfoReply
{
FetchedTagInfo = string.Empty
};
}
}, TaskContinuationOptions.OnlyOnRanToCompletion);
}

Answer 2

问题的核心是需要返回一个值。一个附带的问题是，你不应该使用ContinueWith在所有。如果您将ContinueWith替换为await，那么您的代码将变得更加清晰:

public async override Task<TagInfoReply> GetInfo(TagInfoRequest request, ServerCallContext context)
{
var tagInfo = JsonSerializer.Deserialize<TagInfo>(request.TagInfo);
var parameter = JsonSerializer.Deserialize<ParameterGetWhiteListWithEntryInfo>(request.DetectedTag);
var result = await Task.Run(() =>
{
var filteredTags = filters.FilterTags(tagInfo);
if (antecedent.Result)
{
var reply = await pcs.GetTagInfo(parameter, tagInfo, TagFilterTime);
return new TagInfoReply
{
FetchedTagInfo = reply
};
}
else
{
return new TagInfoReply
{
FetchedTagInfo = string.Empty
};
}
});
return result;
}

你也可能不需要Task.Run在那里:

public async override Task<TagInfoReply> GetInfo(TagInfoRequest request, ServerCallContext context)
{
var tagInfo = JsonSerializer.Deserialize<TagInfo>(request.TagInfo);
var parameter = JsonSerializer.Deserialize<ParameterGetWhiteListWithEntryInfo>(request.DetectedTag);
var filteredTags = filters.FilterTags(tagInfo);
if (antecedent.Result)
{
var reply = await pcs.GetTagInfo(parameter, tagInfo, TagFilterTime);
return new TagInfoReply
{
FetchedTagInfo = reply
};
}
else
{
return new TagInfoReply
{
FetchedTagInfo = string.Empty
};
}
}