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    <p>Does your <code>getB</code> <em>have</em> to be a method which returns <code>Task&lt;B&gt;</code> rather than <code>B</code>?</p> <p>The problem is that <a href="http://msdn.microsoft.com/en-us/library/dd989582.aspx" rel="noreferrer"><code>ContinueWith</code></a> is:</p> <pre><code>public Task&lt;TNewResult&gt; ContinueWith&lt;TNewResult&gt;( Func&lt;Task&lt;TResult&gt;, TNewResult&gt; continuationFunction, CancellationToken cancellationToken ) </code></pre> <p>So in your case, because <code>getB</code> returns <code>Task&lt;B&gt;</code>, you're passing in a <code>Func&lt;Task&lt;A&gt;, Task&lt;B&gt;&gt;</code>, so <code>TNewResult</code> is <code>Task&lt;B&gt;</code>.</p> <p>If you can change <code>getB</code> to just return a <code>B</code> given an <code>A</code>, that would work... or you could use:</p> <pre><code>return ta.ContinueWith(a =&gt; getB(a.Result).Result); </code></pre> <p>Then the lambda expression will be of type, <code>Func&lt;Task&lt;A&gt;, B&gt;</code> so <code>ContinueWith</code> will return a <code>Task&lt;B&gt;</code>.</p> <p>EDIT: In C# 5 you could easily write:</p> <pre><code>public async Task&lt;B&gt; CombinedAsync() { A a = await getA(); B b = await getB(a); return b; } </code></pre> <p>... so it's "just" a matter of working out what that ends up as. I <em>suspect</em> it's something like this, but with error handling:</p> <pre><code>public Task&lt;B&gt; CombinedAsync() { TaskCompletionSource&lt;B&gt; source = new TaskCompletionSource(); getA().ContinueWith(taskA =&gt; { A a = taskA.Result; Task&lt;B&gt; taskB = getB(a); taskB.ContinueWith(t =&gt; source.SetResult(t.Result)); }); return source.Task; } </code></pre> <p>Does that make sense?</p>
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    1. POChaining two functions () -> Task<A> and A->Task<B>
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    1. POChaining two functions () -> Task<A> and A->Task<B>
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    1. CO`getB` doesn't _have_ to return a `Task<B>`, it is just convenient because it represents an asynchronous operation. The problem with `getB(a).Result` is that it will block a thread as soon as the `Task<A>` has completed, so this doesn't scale well in an application where I have thousands of A's and B's.
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    2. CO@BjarkeEbert: Right. It's possible that using a `TaskCompletionSource` would help here. It does feel like the kind of thing that *should* be easy (and actually is with async in C# 5, so I should be able to backport the thinking, as it were...)
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    3. COThanks for the update. I really look forward to C# 5 (in my current project it is not an option) - things are really easier that way. I think the `TaskCompletionSource` method is usable when being non-blocking really matters, and one could even make a generic combinator for combining getA and getB.
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