blocks|key|845537|text|你不能异步调用它，因为没有异步属性支持，只有异步方法。因此，有两种选择，都利用了CTP中的异步方法实际上只是一个返回Task<T>或Task的方法这一事实。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|845538|//+Make+the+property+return+a+Task<T>
public+Task<IEnumerable>+MyList
{
++++get
++++{
+++++++++//+Just+call+the+method
+++++++++return+MyAsyncMethod();
++++}
}|code-block|syntax|javascript|845539|或者：|845540|//+Make+the+property+blocking
public+IEnumerable+MyList
{
++++get
++++{
+++++++++//+Block+via+.Result
+++++++++return+MyAsyncMethod().Result;
++++}
}|845541|entityMap^0|1M|7|1U|4|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

You can't call it asynchronously, since there is no asynchronous property support, only async methods. As such, there are two options, both taking advantage of the fact that asynchronous methods in the CTP are really just a method that returns <code>Task&lt;T&gt;</code> or <code>Task</code>:

<pre><code>// Make the property return a Task&lt;T&gt;
public Task&lt;IEnumerable&gt; MyList
{
 get
 {
 // Just call the method
 return MyAsyncMethod();
 }
}
</code></pre>

Or:

<pre><code>// Make the property blocking
public IEnumerable MyList
{
 get
 {
 // Block via .Result
 return MyAsyncMethod().Result;
 }
}
</code></pre>

blocks|key|630280|text|您可以像这样使用Task：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|630281|public+int+SelectedTab
++++++++{
++++++++++++get+=>+selected_tab;
++++++++++++set
++++++++++++{
++++++++++++++++selected_tab+=+value;

++++++++++++++++new+Task(async+()+=>
++++++++++++++++{
++++++++++++++++++++await+newTab.ScaleTo(0.8);
++++++++++++++++}).Start();
++++++++++++}
++++++++}|code-block|syntax|javascript|630282|entityMap^0|8|4|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

You can use <code>Task</code> like this :

<pre><code>public int SelectedTab
 {
 get =&gt; selected_tab;
 set
 {
 selected_tab = value;

 new Task(async () =&gt;
 {
 await newTab.ScaleTo(0.8);
 }).Start();
 }
 }
</code></pre>

blocks|key|846605|text|我认为我们可以等待值首先返回null，然后获得真正的值，所以在纯MVVM+(例如PCL项目)的情况下，我认为以下是最优雅的解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|846606|private+IEnumerable+myList;
public+IEnumerable+MyList
{
++get
++++{+
++++++if(myList+==+null)
+++++++++InitializeMyList();
++++++return+myList;
+++++}
++set
+++++{
++++++++myList+=+value;
++++++++NotifyPropertyChanged();
+++++}
}

private+async+void+InitializeMyList()
{
+++MyList+=+await+AzureService.GetMyList();
}|code-block|syntax|javascript|846607|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I think that we can await for the value just returning first null and then get the real value, so in the case of Pure MVVM (PCL project for instance) I think the following is the most elegant solution:

<pre><code>private IEnumerable myList;
public IEnumerable MyList
{
 get
 { 
 if(myList == null)
 InitializeMyList();
 return myList;
 }
 set
 {
 myList = value;
 NotifyPropertyChanged();
 }
}

private async void InitializeMyList()
{
 MyList = await AzureService.GetMyList();
}
</code></pre>

blocks|key|629993|text|我以为.GetAwaiter().GetResult()就是这个问题的解决方案，不是吗？例如：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|629994|string+_Title;
public+string+Title
{
++++get
++++{
++++++++if+(_Title+==+null)
++++++++{+++
++++++++++++_Title+=+getTitle().GetAwaiter().GetResult();
++++++++}
++++++++return+_Title;
++++}
++++set
++++{
++++++++if+(value+!=+_Title)
++++++++{
++++++++++++_Title+=+value;
++++++++++++RaisePropertyChanged("Title");
++++++++}
++++}
}|code-block|syntax|javascript|629995|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I thought .GetAwaiter().GetResult() was exactly the solution to this problem, no?
eg:

<pre><code>string _Title;
public string Title
{
 get
 {
 if (_Title == null)
 { 
 _Title = getTitle().GetAwaiter().GetResult();
 }
 return _Title;
 }
 set
 {
 if (value != _Title)
 {
 _Title = value;
 RaisePropertyChanged("Title");
 }
 }
}
</code></pre>

blocks|key|846528|text|由于“异步属性”位于视图模型中，因此可以使用AsyncMVVM|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|846529|class+MyViewModel+:+AsyncBindableBase
{
++++public+string+Title
++++{
++++++++get
++++++++{
++++++++++++return+Property.Get(GetTitleAsync);
++++++++}
++++}

++++private+async+Task<string>+GetTitleAsync()
++++{
++++++++//...
++++}
}|code-block|syntax|javascript|846530|它将为您处理同步上下文和属性更改通知。|846531|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/dmitry-shechtman/AsyncMvvm/^0|M|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

Since your "async property" is in a viewmodel, you could use <a href="https://github.com/dmitry-shechtman/AsyncMvvm/" rel="nofollow">AsyncMVVM</a>:

<pre><code>class MyViewModel : AsyncBindableBase
{
 public string Title
 {
 get
 {
 return Property.Get(GetTitleAsync);
 }
 }

 private async Task&lt;string&gt; GetTitleAsync()
 {
 //...
 }
}
</code></pre>

It will take care of the synchronization context and property change notification for you.

blocks|key|846757|text|亡灵法术。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|846758|在.NET核心/网标2中，您可以使用Nito.AsyncEx.AsyncContext.Run而不是System.Windows.Threading.Dispatcher.InvokeAsync：|offset|length|style|CODE|846759|class+AsyncPropertyTest
{

++++private+static+async+System.Threading.Tasks.Task<int>+GetInt(string+text)
++++{
++++++++await+System.Threading.Tasks.Task.Delay(2000);
++++++++System.Threading.Thread.Sleep(2000);
++++++++return+int.Parse(text);
++++}


++++public+static+int+MyProperty
++++{
++++++++get
++++++++{
++++++++++++int+x+=+0;

++++++++++++//+https://stackoverflow.com/questions/6602244/how-to-call-an-async-method-from-a-getter-or-setter
++++++++++++//+https://stackoverflow.com/questions/41748335/net-dispatcher-for-net-core
++++++++++++//+https://github.com/StephenCleary/AsyncEx
++++++++++++Nito.AsyncEx.AsyncContext.Run(async+delegate+()
++++++++++++{
++++++++++++++++x+=+await+GetInt("123");
++++++++++++});

++++++++++++return+x;
++++++++}
++++}


++++public+static+void+Test()
++++{
++++++++System.Console.WriteLine(System.DateTime.Now.ToString("dd.MM.yyyy+HH:mm:ss.fff"));
++++++++System.Console.WriteLine(MyProperty);
++++++++System.Console.WriteLine(System.DateTime.Now.ToString("dd.MM.yyyy+HH:mm:ss.fff"));
++++}


}|code-block|syntax|javascript|846760|如果您简单地选择System.Threading.Tasks.Task.Run或System.Threading.Tasks.Task<int>.Run，那么它将无法工作。|846761|entityMap^0|0|I|T|1E|1B|0|0|8|V|14|10|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]|$D|U|E|V|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|W|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|X|8|@$D|Y|E|Z|F|G]|$D|10|E|11|F|G]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|P|$]]

Necromancing. 
In .NET Core/NetStandard2, you can use <code>Nito.AsyncEx.AsyncContext.Run</code> instead of <code>System.Windows.Threading.Dispatcher.InvokeAsync</code>: 

<pre><code>class AsyncPropertyTest
{

 private static async System.Threading.Tasks.Task&lt;int&gt; GetInt(string text)
 {
 await System.Threading.Tasks.Task.Delay(2000);
 System.Threading.Thread.Sleep(2000);
 return int.Parse(text);
 }


 public static int MyProperty
 {
 get
 {
 int x = 0;

 // https://stackoverflow.com/questions/6602244/how-to-call-an-async-method-from-a-getter-or-setter
 // https://stackoverflow.com/questions/41748335/net-dispatcher-for-net-core
 // https://github.com/StephenCleary/AsyncEx
 Nito.AsyncEx.AsyncContext.Run(async delegate ()
 {
 x = await GetInt("123");
 });

 return x;
 }
 }


 public static void Test()
 {
 System.Console.WriteLine(System.DateTime.Now.ToString("dd.MM.yyyy HH:mm:ss.fff"));
 System.Console.WriteLine(MyProperty);
 System.Console.WriteLine(System.DateTime.Now.ToString("dd.MM.yyyy HH:mm:ss.fff"));
 }


}
</code></pre>

If you simply chose <code>System.Threading.Tasks.Task.Run</code> or <code>System.Threading.Tasks.Task&lt;int&gt;.Run</code>, then it wouldn't work.

blocks|key|846800|text|您可以创建事件，并在属性更改时调用事件。如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|846801|private+event+EventHandler<string>+AddressChanged;
public+YourClassConstructor(){
+++++AddressChanged+%2B=+GoogleAddressesViewModel_AddressChanged;
}
private+async+void+GoogleAddressesViewModel_AddressChanged(object+sender,+string+e){
++++++...+make+your+async+call
}
private+string+_addressToSearch;
public+string+AddressToSearch
{
++++get+{+return+_addressToSearch;+}
++++set
++++{
++++++++_addressToSearch+=+value;
++++++++AddressChanged.Invoke(this,+AddressToSearch);
++++}
}|code-block|syntax|javascript|846802|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can create an event and invoke an event when the property is changed.
Something like this:
<pre><code>private event EventHandler&lt;string&gt; AddressChanged;
public YourClassConstructor(){
 AddressChanged += GoogleAddressesViewModel_AddressChanged;
}
private async void GoogleAddressesViewModel_AddressChanged(object sender, string e){
 ... make your async call
}
private string _addressToSearch;
public string AddressToSearch
{
 get { return _addressToSearch; }
 set
 {
 _addressToSearch = value;
 AddressChanged.Invoke(this, AddressToSearch);
 }
}
</code></pre>

blocks|key|630267|text|我认为我下面的例子可能遵循@Stephen-Cleary的方法，但我想给出一个编码的例子，这是在数据绑定上下文中使用的，例如Xamarin。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|630268|类的构造函数-或者实际上是它所依赖的另一个属性的设置器-可以调用异步void，该异步void将在任务完成时填充该属性，而不需要等待或阻塞。当它最终获得一个值时，它会通过NotifyPropertyChanged机制更新你的UI。|630269|我不确定从构造函数调用aysnc+void会有什么副作用。也许评论者会详细说明错误处理等。|630270|class+MainPageViewModel+:+INotifyPropertyChanged
{
++++IEnumerable+myList;

++++public+event+PropertyChangedEventHandler+PropertyChanged;

++++public+MainPageViewModel()
++++{

++++++++MyAsyncMethod()

++++}

++++public+IEnumerable+MyList
++++{
++++++++set
++++++++{
++++++++++++if+(myList+!=+value)
++++++++++++{
++++++++++++++++myList+=+value;

++++++++++++++++if+(PropertyChanged+!=+null)
++++++++++++++++{
++++++++++++++++++++PropertyChanged(this,+new+PropertyChangedEventArgs("MyList"));
++++++++++++++++}
++++++++++++}
++++++++}
++++++++get
++++++++{
++++++++++++return+myList;
++++++++}
++++}

++++async+void+MyAsyncMethod()
++++{
++++++++MyList+=+await+DoSomethingAsync();
++++}


}|code-block|syntax|javascript|630271|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I think my example below may follow @Stephen-Cleary 's approach but I wanted to give a coded example. This is for use in a data binding context for example Xamarin.

The constructor of the class - or indeed the setter of another property on which it is dependent - may call an async void that will populate the property on completion of the task without the need for an await or block. When it finally gets a value it will update your UI via the NotifyPropertyChanged mechanism.

I'm not certain about any side effects of calling a aysnc void from a constructor. Perhaps a commenter will elaborate on error handling etc.

<pre><code>class MainPageViewModel : INotifyPropertyChanged
{
 IEnumerable myList;

 public event PropertyChangedEventHandler PropertyChanged;

 public MainPageViewModel()
 {

 MyAsyncMethod()

 }

 public IEnumerable MyList
 {
 set
 {
 if (myList != value)
 {
 myList = value;

 if (PropertyChanged != null)
 {
 PropertyChanged(this, new PropertyChangedEventArgs("MyList"));
 }
 }
 }
 get
 {
 return myList;
 }
 }

 async void MyAsyncMethod()
 {
 MyList = await DoSomethingAsync();
 }


}
</code></pre>

blocks|key|630324|text|我检查了所有答案，但都有性能问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|630325|例如：|630326|string+_Title;
public+string+Title
{
++++get
++++{
++++++++if+(_Title+==+null)
++++++++{+++
++++++++++++Deployment.Current.Dispatcher.InvokeAsync(async+()+=>+{+Title+=+await+getTitle();+});
++++++++}
++++++++return+_Title;
++++}
++++set
++++{
++++++++if+(value+!=+_Title)
++++++++{
++++++++++++_Title+=+value;
++++++++++++RaisePropertyChanged("Title");
++++++++}
++++}
}|code-block|syntax|javascript|630327|Deployment.Current.Dispatcher.InvokeAsync(async+()+=>+{标题=等待getTitle()；})；|630328|使用dispatcher，这不是一个好的答案。|630329|但有一个简单的解决方案，就是这么做：|630330|string+_Title;
++++public+string+Title
++++{
++++++++get
++++++++{
++++++++++++if+(_Title+==+null)
++++++++++++{+++
++++++++++++++++Task.Run(()=>+
++++++++++++++++{
++++++++++++++++++++_Title+=+getTitle();
++++++++++++++++++++RaisePropertyChanged("Title");
++++++++++++++++});++++++++
++++++++++++++++return;
++++++++++++}
++++++++++++return+_Title;
++++++++}
++++++++set
++++++++{
++++++++++++if+(value+!=+_Title)
++++++++++++{
++++++++++++++++_Title+=+value;
++++++++++++++++RaisePropertyChanged("Title");
++++++++++++}
++++++++}
++++}|630331|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

I review all answer but all have a performance issue.

for example in :

<pre><code>string _Title;
public string Title
{
 get
 {
 if (_Title == null)
 { 
 Deployment.Current.Dispatcher.InvokeAsync(async () =&gt; { Title = await getTitle(); });
 }
 return _Title;
 }
 set
 {
 if (value != _Title)
 {
 _Title = value;
 RaisePropertyChanged("Title");
 }
 }
}
</code></pre>

Deployment.Current.Dispatcher.InvokeAsync(async () => { Title = await getTitle(); });

use dispatcher which is not a good answer.

but there is a simple solution, just do it:

<pre><code>string _Title;
 public string Title
 {
 get
 {
 if (_Title == null)
 { 
 Task.Run(()=&gt; 
 {
 _Title = getTitle();
 RaisePropertyChanged("Title");
 }); 
 return;
 }
 return _Title;
 }
 set
 {
 if (value != _Title)
 {
 _Title = value;
 RaisePropertyChanged("Title");
 }
 }
 }
</code></pre>

blocks|key|846839|text|你可以继续使用它|type|unstyled|depth|inlineStyleRanges|entityRanges|data|846840|++++async+Task<IEnumerable>+MyAsyncMethod()
++++{
++++++++return+await+DoSomethingAsync();
++++}

++++public+IEnumerable+MyList
++++{
++++++++get
++++++++{
++++++++++++MyAsyncMethod()
++++++++++++++.ContinueWith((tsk)+=>
++++++++++++++{
++++++++++++++++++//+what+every+work+
++++++++++++++});
++++++++++return+_myList
++++++++}
++++}|code-block|syntax|javascript|846841|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can use it continuation
<pre><code> async Task&lt;IEnumerable&gt; MyAsyncMethod()
 {
 return await DoSomethingAsync();
 }

 public IEnumerable MyList
 {
 get
 {
 MyAsyncMethod()
 .ContinueWith((tsk) =&gt;
 {
 // what every work 
 });
 return _myList
 }
 }
</code></pre>

blocks|key|846655|text|您可以将proerty更改为Task<IEnumerable>|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|846656|然后做一些类似的事情：|846657|get
{
++++Task<IEnumerable>.Run(async()=>{
+++++++return+await+getMyList();
++++});
}|code-block|syntax|javascript|846658|并像等待MyList一样使用它；|846659|entityMap^0|E|H|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|V|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|P|$]]

You can change the proerty to <code>Task&lt;IEnumerable&gt;</code>

and do something like:

<pre><code>get
{
 Task&lt;IEnumerable&gt;.Run(async()=&gt;{
 return await getMyList();
 });
}
</code></pre>

and use it like
await MyList;

What'd be the most elegant way to call an async method from a getter or setter in C#?

Here's some pseudo-code to help explain myself.

<pre><code>async Task&lt;IEnumerable&gt; MyAsyncMethod()
{
 return await DoSomethingAsync();
}

public IEnumerable MyList
{
 get
 {
 //call MyAsyncMethod() here
 }
}
</code></pre>

How to call an async method from a getter or setter?

Windows

Xamarin

在C#中从getter或setter调用异步方法最优雅的方式是什么？这里有一些伪代码来帮助我解释。async Task<IEnumerable> MyAsyncMethod(){ return await DoSomethingAsync();}public IEnumerable MyList{ get { //call MyAsyncMethod() here

问如何从getter或setter调用异步方法？
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问如何从getter或setter调用异步方法？
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