Lazy load properties with Async

Question

I've learned to lazy load properties in my repository. Now I'd like to do that, but I also need to load something from a web page (using Httpclient) which means my property will be async.

public async Task<List<NewsModel>> News
{
    get
    {
        if (_news == null)
        {
            CacheProvider cache = new CacheProvider();
            object cachedNews = cache.Get("news");

            if (cachedNews == null)
            {
                var client = new HttpClient();
                // await HttpResponse
            }

        }
        return _news;
    }

    set
    {
        _news = value;
    }
}

However, visual studio is telling me that

"The modifier async is not valid for this item"

while highlighting the word "News" in the first line.

Is it possible to do this? Or do I have to write a separate method?

I take it that you've got the using statement for System.Threading.Tasks? Also, which .NET framework is this targeting? Is it Silverlight? -- Never mind, just saw that this is a property. You'll want to convert this into methods. — Ben H
– Ben H, Commented Jul 31, 2013 at 16:21
It seems to me like the property News would intuitively have the type List<NewsModel>, the Task<> decoration doesn't seem natural here. Why do you feel that you need to expose the Task<> nature? — alc
– alc, Commented Jul 31, 2013 at 16:21
@AndrewCoonce Actually, if he wants to write an async method, he would definitely have to write it as returning a task. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Jul 31, 2013 at 16:22
Because visual studio told me to do that :( First time I deal with async in C#. — Saturnix
– Saturnix, Commented Jul 31, 2013 at 16:22
@LasseV.Karlsen: Yes, but this isn't a method... this is a property. I guess my concern is that the end-user isn't expecting side-effects, asynchronous operation, etc... they're expecting an enumeration of news items. — alc
– alc, Commented Jul 31, 2013 at 16:25

Stephen Cleary · Accepted Answer · 2013-07-31 16:32:17Z

14

Asynchronous properties are not supported. I describe a number of workarounds on my blog.

In your case, it sounds like asynchronous lazy initialization would be a good solution (also described on my blog).

answered Jul 31, 2013 at 16:32

Stephen Cleary

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Lasse V. Karlsen · Accepted Answer · 2013-07-31 16:28:39Z

9

First of all, properties with side-effects are usually not all that good.

In this case, simply reading this property would kick off a thread, some network traffic, and some processing on a remote server.

This should be a method, not a property.

Secondly, the compiler is right, properties are not allowed to be asynchronous. Now, you can definitely write a property that returns an asynchronous task, but you're not allowed to use the async keyword. Basically just take away the async keyword from the property declaration.

But the fact that async is not legal on properties is another clue that you should write a method, not a property.

Note The async keyword is not actually required in the code you've posted, since you're not actually using the await keyword in there. As such, you can simply remove the async keyword altogether, since it is not required. In fact, if you were to change this over to a method, the compiler will tell you that it is unnecessary since you're not using await. (Removed after OP edited the question.)

edited Jul 31, 2013 at 16:28

answered Jul 31, 2013 at 16:21

Lasse V. Karlsen

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9 Comments

Saturnix Over a year ago

oh wait! I am using await! httpclient will use await as the guy here did: stackoverflow.com/questions/7929013/…

Saturnix Over a year ago

If I remove the 'await' I have to wrap HttpResponseMessage in a task - which means I have to write an 'await' somewhere in the code...

Lasse V. Karlsen Over a year ago

Well, you still cannot write properties using async, so await has to go as well.

user585968 Over a year ago

"First of all, properties with side-effects are usually not all that good" - well no, that's how proxy classes with on-demand/lazy-load loading work such as in ORMs

Hypnovirus Over a year ago

I agree with "properties with side-effects are usually not all that good" but I don't think it is applicable here. Properties should not change the logical state of the system. I don't think loading data is a side-effect. Querying a property is asking a question. Working to answer the question is an expected effect.

|

Artem · Accepted Answer · 2023-09-11 20:14:30Z

You can use a Lazy property that returns a Task:

class MyClass
{
    readonly Lazy<Task<string>> _text;

    public MyClass()
    {
        _text = new Lazy<Task<string>>(async () =>
        {
            //... await something
            return "Hello!"
        });
    }
 
    async Task DoSomething()
    {
       var text = await _text.Value;
       //...
    }
}

Update: Be aware that Lazy uses Monitor.Enter internally, so accessing it from many threads simultaneously can cause a denial of service. I have made a basic implementation of Lazy using SemaphoreSlim, which will leave threads available while waiting for the value. See my gist here

IMHO this make Lazy useless, in this case Lazy will only initialize task, which most of the cases is not what you want

Community · Accepted Answer · 2017-05-23 11:53:29Z

1

I think this question is related.

In short - async properties are not supported.

edited May 23, 2017 at 11:53

CommunityBot

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answered Jul 31, 2013 at 16:22

MBender

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