C#AsyncPatterns.cs
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using System;
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using System.Collections.Generic;
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using System.Net.Http;
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using System.Threading;
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using System.Threading.Tasks;
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public class AsyncPatterns
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{
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    private static readonly HttpClient _http = new();
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    // Pattern 1: Concurrent requests with Task.WhenAll
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    public static async Task<string[]> FetchAllAsync(
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        IEnumerable<string> urls,
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        CancellationToken ct = default)
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    {
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        var tasks = urls.Select(url => FetchAsync(url, ct));
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        return await Task.WhenAll(tasks).ConfigureAwait(false);
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    }
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    // Pattern 2: ConfigureAwait(false) in library code
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    private static async Task<string> FetchAsync(string url, CancellationToken ct)
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    {
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        using var response = await _http.GetAsync(url, ct).ConfigureAwait(false);
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        response.EnsureSuccessStatusCode();
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        return await response.Content.ReadAsStringAsync(ct).ConfigureAwait(false);
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    }
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    // Pattern 3: IAsyncEnumerable for streaming results
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    public static async IAsyncEnumerable<string> StreamResultsAsync(
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        IEnumerable<string> urls,
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        [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken ct = default)
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    {
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        foreach (var url in urls)
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        {
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            ct.ThrowIfCancellationRequested();
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            string result = await FetchAsync(url, ct).ConfigureAwait(false);
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            yield return result;
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        }
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    }
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    // Pattern 4: Timeout via CancellationTokenSource
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    public static async Task RunWithTimeoutAsync(Func<CancellationToken, Task> work)
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    {
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        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
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        try
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        {
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            await work(cts.Token).ConfigureAwait(false);
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        }
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        catch (OperationCanceledException)
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        {
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            Console.WriteLine("Operation timed out after 5 seconds.");
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        }
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    }
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}
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