使用可移植类库进行异步单元测试

Question

我有一个至少需要针对 .net 4.5 和 Silverlight 5 的可移植类库。我在尝试在 VS 2012 中编写 MSTest 单元测试时遇到了问题，因为我的库不使用新的 async/await 范例.有什么方法可以测试这个方法吗？

public static void Get(string uri, string acceptHeader, Action<string> callback)
{
    var request = (HttpWebRequest)WebRequest.Create(uri);
    request.Accept = acceptHeader;        

    request.BeginGetResponse(o =>
    {
        var r = o.AsyncState as HttpWebRequest;
        try
        {
            var response = r.EndGetResponse(o);
            using (var sr = new StreamReader(response.GetResponseStream()))
            {
                callback(sr.ReadToEnd());
            }
        }
        catch (Exception ex)
        {
            throw new WebException(string.Format("Unable to access {0}", uri), ex);
        }
    }, request);
}

Answer 1

首先，我建议你重新考虑async/await。这是未来的浪潮。 Microsoft.Bcl.Async 为面向 .NET 4.5 和 SL5 的可移植库提供 async 支持。

但如果你不想这样做，你仍然可以使用async 单元测试：

[TestMethod]
public async Task Get_RetrievesExpectedString()
{
  var tcs = new TaskCompletionSource<string>();
  var client = new ... // arrange

  client.Get(uri, acceptHeader, result =>
  {
    tcs.SetResult(result);
  });
  var actual = await tcs.Task;

  Assert.AreEqual(expected, actual);
}

或者，如果你愿意，你也可以“老派”：

[TestMethod]
public void Get_RetrievesExpectedString()
{
  var mre = new ManualResetEvent(initialState: false);
  string actual = null;
  var client = new ... // arrange

  client.Get(uri, acceptHeader, result =>
  {
    actual = result;
    mre.Set();
  });
  mre.WaitOne();

  Assert.AreEqual(expected, actual);
}

Answer 2

无法抗拒重构代码。您可以使用闭包来执行以下操作

public static void Get(string uri, string acceptHeader, Action<string> callback)
{
    var request = (HttpWebRequest)WebRequest.Create(uri);
    request.Accept = acceptHeader;        

    request.BeginGetResponse(o =>
    {
        try
        {
            var response = request.EndGetResponse(o);
            using (var sr = new StreamReader(response.GetResponseStream()))
            {
                callback(sr.ReadToEnd());
            }
        }
        catch (Exception ex)
        {
            throw new WebException(string.Format("Unable to access {0}", uri), ex);
        }
    }, null);
}

不过，最终您可以执行以下操作

public async static void Get(string uri, string acceptHeader, Action<string> callback)
{
    var request = (HttpWebRequest)WebRequest.Create(uri);
    request.Accept = acceptHeader;        
    var response = await Task.Factory.FromAsync(
                             request.BeginGetRequestStream ,
                             request.EndGetRequestStream ,
                             uri,
                             null);
    using (var sr = new StreamReader(response))
    {
        callback(sr.ReadToEnd());
    }
}

好的，这就是我的做法

void Main()
{
    {...}
    var request = (HttpWebRequest)WebRequest.Create(uri);
    request.Accept = acceptHeader;
    var response = await request.DownloadStringTaskAwait();
    DoSomeStuff(response);
}

// Define other methods and classes here
public static class HttpWebRequestExtension
{
    public async Task<string> DownloadStringTaskAwait(this HttpWebRequest request)
    {
        var response = await Task.Factory.FromAsync<Stream>(
                            request.BeginGetRequestStream,
                            request.EndGetRequestStream,
                            null);
                    using (var sr = new StreamReader(response))
                    {
                         return sr.ReadToEnd();
                    }
            }

}