| title | description | ms.date | dev_langs | helpviewer_keywords | ms.assetid | |||
|---|---|---|---|---|---|---|---|---|
Create pre-computed Task objects |
In this article, you'll learn how to create pre-computed tasks. |
05/04/2021 |
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a73eafa2-1f49-4106-a19e-997186029b58 |
In this article, you'll learn how to use the xref:System.Threading.Tasks.Task.FromResult%2A?displayProperty=nameWithType method to retrieve the results of asynchronous download operations that are held in a cache. The xref:System.Threading.Tasks.Task.FromResult%2A method returns a finished xref:System.Threading.Tasks.Task%601 object that holds the provided value as its xref:System.Threading.Tasks.Task%601.Result%2A property. This method is useful when you perform an asynchronous operation that returns a xref:System.Threading.Tasks.Task%601 object, and the result of that xref:System.Threading.Tasks.Task%601 object is already computed.
The following example downloads strings from the web. It defines the DownloadStringAsync method. This method downloads strings from the web asynchronously. This example also uses a xref:System.Collections.Concurrent.ConcurrentDictionary%602 object to cache the results of previous operations. If the input address is held in this cache, DownloadStringAsync uses the xref:System.Threading.Tasks.Task.FromResult%2A method to produce a xref:System.Threading.Tasks.Task%601 object that holds the content at that address. Otherwise, DownloadStringAsync downloads the file from the web and adds the result to the cache.
:::code language="csharp" source="snippets/cs/DownloadCache.cs"::: :::code language="vb" source="snippets/vb/DownloadCache.vb":::
In the preceding example, the first time each url is downloaded, its value is stored in the cache. The xref:System.Threading.Tasks.Task.FromResult%2A method enables the DownloadStringAsync method to create xref:System.Threading.Tasks.Task%601 objects that hold these pre-computed results. Subsequent calls to download the string return the cached values, and is much faster.