如何在 Polly ExecuteAsync 的 lambda 表达式中包含多行代码块？

Question

我有使用 Polly 策略 (IAsyncPolicy<HttpResponseMessage>) 上的 ExecuteAsync() 方法从网页提取数据的工作代码。我想向它添加一些调试日志记录，但到目前为止我无法找出正确的语法来实现这种情况，除非我将日志记录和 http 调用包装在一个单独的方法中，然后通过 ExecuteAsync.

这对我来说似乎不“正确”，我觉得有更简洁或更直接的方法。我认为我的问题是我对 Func<T> 和 lambda 表达式如何真正运作以实现这一点没有足够完整的理解，因为当我将内容包装在 {} 中时，我得到如下错误：

Not all code paths return a value in lambda expression of type Func<CancellationToken, Task<HttpResponseMessage>>.

这是我原来的工作代码：

var httpClient = _httpClientFactory.CreateClient();
HttpResponseMessage result;
var retryPolicy = (IAsyncPolicy<HttpResponseMessage>)_policyRegistry[RetryPolicyKeyName];

result = await retryPolicy.ExecuteAsync(
    async cancellationToken =>    
        await httpClient.GetAsync(myUrl, cancellationToken)
    , cancellationToken
);

此代码产生“并非所有代码路径 return 类型为 Func<CancellationToken, Task> 的 lambda 表达式中的值错误：

var httpClient = _httpClientFactory.CreateClient();
HttpResponseMessage result;
var retryPolicy = (IAsyncPolicy<HttpResponseMessage>)_policyRegistry[RetryPolicyKeyName];

result = await retryPolicy.ExecuteAsync(
    async cancellationToken =>    
    {
        await httpClient.GetAsync(myUrl, cancellationToken);
    }
    , cancellationToken
);

这是我的“hack”，它通过额外的方法完成了我想要的事情：

private async Task<HttpResponseMessage> TestWithLogAsync(HttpClient httpClient, string myUrl, CancellationToken cancellationToken)
{
    _logger.LogDebug("Requesting from: {myUrl}", myUrl);
    var result = await httpClient.GetAsync(myUrl, cancellationToken);
    return result;
}

var httpClient = _httpClientFactory.CreateClient();
HttpResponseMessage result;
var retryPolicy = (IAsyncPolicy<HttpResponseMessage>)_policyRegistry[RetryPolicyKeyName];

result = await retryPolicy.ExecuteAsync(
    async cancellationToken =>
        await TestWithLogAsync(httpClient, myUrl, cancellationToken)
    , cancellationToken
);

Answer 1

你这里有：

result = await retryPolicy.ExecuteAsync(
    await httpClient.GetAsync(managementUrl, cancellationToken)
    , cancellationToken
);

相当于：

result = await retryPolicy.ExecuteAsync(async () => {
      return await httpClient.GetAsync(managementUrl, cancellationToken);
    }
    , cancellationToken
);

因此这将在 lambda 中执行多行：

result = await retryPolicy.ExecuteAsync(async () => {
      _logger.LogDebug("Requesting from: {managementUrl}", managementUrl);
      return await httpClient.GetAsync(managementUrl, cancellationToken);
    }
    , cancellationToken
);

_{..虽然我不太清楚managementUrl如何在你的例子中变成myUrl，以及它们是否不同..我假设没有}

如何在 Polly ExecuteAsync 的 lambda 表达式中包含多行代码块？

How to include a multiline block of code in a lambda expression for Polly ExecuteAsync?

c#

lambda

polly