Retry with Exponential Backoff

// Demo: the operation fails twice, then succeeds, so RetryAsync makes three calls.
int calls = 0;

string result = await RetryHelper.RetryAsync(async () =>
{
    calls++;
    await Task.Delay(10);
    if (calls < 3) throw new HttpRequestException("temporary failure");
    return "ok";
}, maxAttempts: 4, initialDelay: TimeSpan.FromMilliseconds(10));

Console.WriteLine($"{result} after {calls} calls");     // ok after 3 calls

// Real use: await RetryHelper.RetryAsync(() => http.GetStringAsync(url), maxAttempts: 4);

public static class RetryHelper
{
    // Waits initialDelay, then twice as long after each failure. The last failure is rethrown.
    public static async Task<T> RetryAsync<T>(
        Func<Task<T>> action,
        int maxAttempts = 3,
        TimeSpan? initialDelay = null,
        CancellationToken ct = default)
    {
        var delay = initialDelay ?? TimeSpan.FromMilliseconds(200);

        for (var attempt = 1; ; attempt++)
        {
            try
            {
                return await action();
            }
            catch (Exception) when (attempt < maxAttempts && !ct.IsCancellationRequested)
            {
                await Task.Delay(delay, ct);
                delay *= 2;
            }
        }
    }
}