创建预先计算的任务
本文介绍如何使用 Task.FromResult 方法检索缓存中包含的异步下载操作的结果。 FromResult 方法返回一个将所提供的值作为其 Task<TResult> 属性的已完成的 Result 对象。 执行返回 Task<TResult> 对象的异步运算，且已计算该 Task<TResult> 对象的结果时，此方法将十分有用。
示例
下面的示例从 Web 下载字符串。 它定义了 DownloadStringAsync 方法。 此方法从 Web 异步下载字符串。 此示例还使用 ConcurrentDictionary<TKey,TValue> 对象来缓存先前操作的结果。 如果此缓存中包含输入的地址，DownloadStringAsync 会使用 FromResult 方法来生成包含位于该地址的内容的 Task<TResult> 对象。 否则，DownloadStringAsync 从 Web 下载文件并将结果添加到缓存中。
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;
public static class DownloadCache
{
private static readonly ConcurrentDictionary<string, string> s_cachedDownloads = new();
private static readonly HttpClient s_httpClient = new();
public static Task<string> DownloadStringAsync(string address)
{
if (s_cachedDownloads.TryGetValue(address, out string? content))
{
return Task.FromResult(content);
}
return Task.Run(async () =>
{
content = await s_httpClient.GetStringAsync(address);
s_cachedDownloads.TryAdd(address, content);
return content;
});
}
public static async Task Main()
{
string[] urls = new[]
{
"https://learn.microsoft.com/aspnet/core",
"https://learn.microsoft.com/dotnet",
"https://learn.microsoft.com/dotnet/architecture/dapr-for-net-developers",
"https://learn.microsoft.com/dotnet/azure",
"https://learn.microsoft.com/dotnet/desktop/wpf",
"https://learn.microsoft.com/dotnet/devops/create-dotnet-github-action",
"https://learn.microsoft.com/dotnet/machine-learning",
"https://learn.microsoft.com/xamarin",
"https://dotnet.microsoft.com/",
"https://www.microsoft.com"
};
Stopwatch stopwatch = Stopwatch.StartNew();
IEnumerable<Task<string>> downloads = urls.Select(DownloadStringAsync);
static void StopAndLogElapsedTime(
int attemptNumber, Stopwatch stopwatch, Task<string[]> downloadTasks)
{
stopwatch.Stop();
int charCount = downloadTasks.Result.Sum(result => result.Length);
long elapsedMs = stopwatch.ElapsedMilliseconds;
Console.WriteLine(
$"Attempt number: {attemptNumber}\n" +
$"Retrieved characters: {charCount:#,0}\n" +
$"Elapsed retrieval time: {elapsedMs:#,0} milliseconds.\n");
}
await Task.WhenAll(downloads).ContinueWith(
downloadTasks => StopAndLogElapsedTime(1, stopwatch, downloadTasks));
// Perform the same operation a second time. The time required
// should be shorter because the results are held in the cache.
stopwatch.Restart();
downloads = urls.Select(DownloadStringAsync);
await Task.WhenAll(downloads).ContinueWith(
downloadTasks => StopAndLogElapsedTime(2, stopwatch, downloadTasks));
}
// Sample output:
// Attempt number: 1
// Retrieved characters: 754,585
// Elapsed retrieval time: 2,857 milliseconds.
// Attempt number: 2
// Retrieved characters: 754,585
// Elapsed retrieval time: 1 milliseconds.
}
Imports System.Collections.Concurrent
Imports System.Net.Http
Module Snippets
Class DownloadCache
Private Shared ReadOnly s_cachedDownloads As ConcurrentDictionary(Of String, String) =
New ConcurrentDictionary(Of String, String)()
Private Shared ReadOnly s_httpClient As HttpClient = New HttpClient()
Public Function DownloadStringAsync(address As String) As Task(Of String)
Dim content As String = Nothing
If s_cachedDownloads.TryGetValue(address, content) Then
Return Task.FromResult(Of String)(content)
End If
Return Task.Run(
Async Function()
content = Await s_httpClient.GetStringAsync(address)
s_cachedDownloads.TryAdd(address, content)
Return content
End Function)
End Function
End Class
Public Sub StopAndLogElapsedTime(
attemptNumber As Integer,
stopwatch As Stopwatch,
downloadTasks As Task(Of String()))
stopwatch.Stop()
Dim charCount As Integer = downloadTasks.Result.Sum(Function(result) result.Length)
Dim elapsedMs As Long = stopwatch.ElapsedMilliseconds
Console.WriteLine(
$"Attempt number: {attemptNumber}{vbCrLf}" &
$"Retrieved characters: {charCount:#,0}{vbCrLf}" &
$"Elapsed retrieval time: {elapsedMs:#,0} milliseconds.{vbCrLf}")
End Sub
Sub Main()
Dim cache As DownloadCache = New DownloadCache()
Dim urls As String() = {
"https://learn.microsoft.com/aspnet/core",
"https://learn.microsoft.com/dotnet",
"https://learn.microsoft.com/dotnet/architecture/dapr-for-net-developers",
"https://learn.microsoft.com/dotnet/azure",
"https://learn.microsoft.com/dotnet/desktop/wpf",
"https://learn.microsoft.com/dotnet/devops/create-dotnet-github-action",
"https://learn.microsoft.com/dotnet/machine-learning",
"https://learn.microsoft.com/xamarin",
"https://dotnet.microsoft.com/",
"https://www.microsoft.com"
}
Dim stopwatch As Stopwatch = Stopwatch.StartNew()
Dim downloads As IEnumerable(Of Task(Of String)) =
urls.Select(AddressOf cache.DownloadStringAsync)
Task.WhenAll(downloads).ContinueWith(
Sub(downloadTasks)
StopAndLogElapsedTime(1, stopwatch, downloadTasks)
End Sub).Wait()
stopwatch.Restart()
downloads = urls.Select(AddressOf cache.DownloadStringAsync)
Task.WhenAll(downloads).ContinueWith(
Sub(downloadTasks)
StopAndLogElapsedTime(2, stopwatch, downloadTasks)
End Sub).Wait()
End Sub
' Sample output:
' Attempt number 1
' Retrieved characters: 754,585
' Elapsed retrieval time: 2,857 milliseconds.
'
' Attempt number 2
' Retrieved characters: 754,585
' Elapsed retrieval time: 1 milliseconds.
End Module
在上一示例中，首次下载每个 URL 时，其值存储在缓存中。 FromResult 方法可以使 DownloadStringAsync 方法创建包含这些预计算结果的 Task<TResult> 对象。 下载字符串的后续调用将返回缓存的值，并且速度要快得多。
