blocks|key|2912737|text|异步lambda表达式不能转换为简单的Func<TSource,+TResult>。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2912738|因此，选择多个不能使用。您可以在同步上下文中运行：|2912739|myEnumerable.Select(c+=>+Functions.GetDataAsync(c.Id)).SelectMany(task+=>+task.Result);|code-block|syntax|javascript|2912740|或|2912741|List<DataItem>+result+=+new+List<DataItem>();

foreach+(var+ele+in+myEnumerable)
{
++++result.AddRange(await+Functions.GetDataAsyncDo(ele.Id));
}|2912742|你既不能使用yield+return，也不能使用它--它是设计出来的。f.e.：|2912743|public+async+Task<IEnuemrable<DataItem>>+Do()+
{
++++...
++++foreach+(var+ele+in+await+Functions.GetDataAsyncDo(ele.Id))+
++++{
++++++++yield+return+ele;+//+compile+time+error,+async+method+
++++++++++++++++++++++++++//+cannot+be+used+with+yield+return
++++}

}|2912744|entityMap^0|J|M|0|0|0|0|0|6|C|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Z|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|11|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|13|8|@$9|14|A|15|B|C]]|D|@]|E|$]]|$1|S|3|T|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|V|$]]

async lambda expression cannot be converted to simple <code>Func&lt;TSource, TResult&gt;</code>.

So, select many cannot be used. You can run in synchronized context:

<pre><code>myEnumerable.Select(c =&gt; Functions.GetDataAsync(c.Id)).SelectMany(task =&gt; task.Result);
</code></pre>

or

<pre><code>List&lt;DataItem&gt; result = new List&lt;DataItem&gt;();

foreach (var ele in myEnumerable)
{
 result.AddRange(await Functions.GetDataAsyncDo(ele.Id));
}
</code></pre>

You cannot neither use <code>yield return</code> - it is by design. f.e.:

<pre><code>public async Task&lt;IEnuemrable&lt;DataItem&gt;&gt; Do() 
{
 ...
 foreach (var ele in await Functions.GetDataAsyncDo(ele.Id)) 
 {
 yield return ele; // compile time error, async method 
 // cannot be used with yield return
 }

}
</code></pre>

blocks|key|3381033|text|这是一个扩展：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3381034|public+static+async+Task<IEnumerable<T1>>+SelectManyAsync<T,+T1>(this+IEnumerable<T>+enumeration,+Func<T,+Task<IEnumerable<T1>>>+func)
{
++++return+(await+Task.WhenAll(enumeration.Select(func))).SelectMany(s+=>+s);
}|code-block|syntax|javascript|3381035|它允许您运行：|3381036|var+result+=+await+myEnumerable.SelectManyAsync(c+=>+Functions.GetDataAsync(c.Id));|3381037|说明:您有一个任务列表，每个任务都返回Task<IEnumerable<T>>。因此，您需要解雇他们，然后等待所有，然后通过SelectMany压扁结果。|offset|length|style|CODE|3381038|entityMap^0|0|0|0|0|J|K|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@$M|X|N|Y|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

This is an extension:

<pre><code>public static async Task&lt;IEnumerable&lt;T1&gt;&gt; SelectManyAsync&lt;T, T1&gt;(this IEnumerable&lt;T&gt; enumeration, Func&lt;T, Task&lt;IEnumerable&lt;T1&gt;&gt;&gt; func)
{
 return (await Task.WhenAll(enumeration.Select(func))).SelectMany(s =&gt; s);
}
</code></pre>

That allows you to run:

<pre><code>var result = await myEnumerable.SelectManyAsync(c =&gt; Functions.GetDataAsync(c.Id));
</code></pre>

Explanation: you have a list of tasks, each returns <code>Task&lt;IEnumerable&lt;T&gt;&gt;</code>. So you need to fire them all, then await all, and then squash the result via SelectMany.

blocks|key|1647713|text|Select之所以工作，是因为它将返回一个IEnumerable<Task<T>>，然后可以与Task.WhenAll一起等待。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1647714|因此，解决这个问题的一个简单方法是：|1647715|IEnumerable<Task<IEnumerable<T>>>+tasks+=+source.Select(GetNestedEnumerableTask);
IEnumerable<T>[]+nestedResults+=+await+Task.WhenAll(tasks);
IEnumerable<T>+results+=+nestedResults.SelectMany(nr+=>+nr);|code-block|syntax|javascript|1647716|entityMap^0|0|6|L|K|1B|C|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

<code>Select</code> works because it will return an <code>IEnumerable&lt;Task&lt;T&gt;&gt;</code>, which can then be awaited with e.g. <code>Task.WhenAll</code>.

So, an easy workaround to this problem is:

<pre><code>IEnumerable&lt;Task&lt;IEnumerable&lt;T&gt;&gt;&gt; tasks = source.Select(GetNestedEnumerableTask);
IEnumerable&lt;T&gt;[] nestedResults = await Task.WhenAll(tasks);
IEnumerable&lt;T&gt; results = nestedResults.SelectMany(nr =&gt; nr);
</code></pre>

blocks|key|2912932|text|使用C#8和IAsyncEnumerable，我们可以更自然地编写：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2912933|public+static+async+IAsyncEnumerable<R>+SelectManyAsync<T,+R>(this+IEnumerable<T>+ts,+
++++++++++++++++++++++++++++++++++++++++++++++++++++Func<T,+Task<IEnumerable<R>>>+func)
{
++++foreach+(var+t+in+ts)
++++{
++++++++var+rs+=+await+func(t);
++++++++foreach+(var+r+in+rs)
++++++++++++yield+return+r;
++++}
}|code-block|syntax|javascript|2912934|注意:使用await+foreach(...在IAsyncEnumerable上迭代|2912935|entityMap^0|6|G|0|0|5|H|N|G|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

With C#8 and <code>IAsyncEnumerable</code> we can write this more naturally:
<pre><code>public static async IAsyncEnumerable&lt;R&gt; SelectManyAsync&lt;T, R&gt;(this IEnumerable&lt;T&gt; ts, 
 Func&lt;T, Task&lt;IEnumerable&lt;R&gt;&gt;&gt; func)
{
 foreach (var t in ts)
 {
 var rs = await func(t);
 foreach (var r in rs)
 yield return r;
 }
}
</code></pre>
Note:
Use <code>await foreach(...</code> to iterate over an <code>IAsyncEnumerable</code>

I'm getting the following error when trying to use an <code>async</code> lambda within <code>IEnumerable.SelectMany</code>:

<pre><code>var result = myEnumerable.SelectMany(async (c) =&gt; await Functions.GetDataAsync(c.Id));
</code></pre>

<blockquote>
 The type arguments for method 'IEnumerable
 System.Linq.Enumerable.SelectMany(this
 IEnumerable, Func>)' cannot be
 inferred from the usage. Try specifying the type arguments explicitly
</blockquote>

Where <code>GetDataAsync</code> is defined as:

<pre><code>public interface IFunctions {
 Task&lt;IEnumerable&lt;DataItem&gt;&gt; GetDataAsync(string itemId);
}

public class Functions : IFunctions {
 public async Task&lt;IEnumerable&lt;DataItem&gt;&gt; GetDataAsync(string itemId) {
 // return await httpCall();
 }
}
</code></pre>

I guess because my <code>GetDataAsync</code> method actually returns a <code>Task&lt;IEnumerable&lt;T&gt;&gt;</code>. But why does <code>Select</code> work, surely it should throw the same error?

<pre><code>var result = myEnumerable.Select(async (c) =&gt; await Functions.GetDataAsync(c.Id));
</code></pre>

Is there any way around this?

How to use async lambda with SelectMany?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在尝试在async中使用IEnumerable.SelectMany lambda时，我得到了以下错误var result = myEnumerable.SelectMany(async (c) => await Functions.GetDataAsync(c.Id));不能从用法中推断方法“IEnumerable ...

问如何在SelectMany中使用异步lambda？
EN

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问如何在SelectMany中使用异步lambda？EN