在操作方法中抛出 HttpResponseException 时处理 HttpResponseMessage 及其内容

Question

我的问题的来源是以下代码，它是 Microsoft documentation 中包含的代码片段的一部分，用于 asp.net web api 中的异常处理：

var resp = new HttpResponseMessage(HttpStatusCode.NotFound)
{
    Content = new StringContent(string.Format("No product with ID = {0}", id)),
    ReasonPhrase = "Product ID Not Found"
};
throw new HttpResponseException(resp);

HttpResponseMessage和StringContent都实现了IDisposable接口，但是上面代码中没有调用方法IDisposable.Dispose.
这是个问题吗？不处理这些对象是否有任何副作用？

根据 this article 的说法，可能的解决方案是将上述代码更改为以下内容：

var content = new StringContent(string.Format("No product with ID = {0}", id));
var resp = new HttpResponseMessage(HttpStatusCode.NotFound)
{
    Content = content,
    ReasonPhrase = "Product ID Not Found"
};

this.Request.RegisterForDispose(content);
this.Request.RegisterForDispose(resp);

throw new HttpResponseException(resp);

这是否真的有必要，或者是否有可能避免这种情况（根据 Microsoft 文档中显示的内容）？

Answer 1

检查Microsoft Source for HttpResponseMessage.CS：

protected virtual void Dispose(bool disposing)
{
    // The reason for this type to implement IDisposable is that it contains instances of 
    // types that implement IDisposable (content). 
    if (disposing && !_disposed)
    {
        _disposed = true;
        if (_content != null)
        {
            _content.Dispose();
        }
    }
}

content 是 HttpContent 类型。检查 Microsoft Source for HttpContent.cs:

protected override void Dispose(bool disposing)
{
    Debug.Assert(_buffer != null);

    ArrayPool<byte>.Shared.Return(_buffer);
    _buffer = null;

    base.Dispose(disposing);
}

ArrayPool 的评论说：

/// Renting and returning buffers with an <see cref="ArrayPool{T}"/> can increase performance
/// in situations where arrays are created and destroyed frequently, resulting in significant
/// memory pressure on the garbage collector.

检查 ArrayPool 的源代码会产生这个可爱的 gem:

    /// <summary>
    /// Retrieves a shared <see cref="ArrayPool{T}"/> instance.
    /// </summary>
    /// <remarks>
    /// The shared pool provides a default implementation of <see cref="ArrayPool{T}"/>
    /// that's intended for general applicability.  It maintains arrays of multiple sizes, and 
    /// may hand back a larger array than was actually requested, but will never hand back a smaller 
    /// array than was requested. Renting a buffer from it with <see cref="Rent"/> will result in an 
    /// existing buffer being taken from the pool if an appropriate buffer is available or in a new 
    /// buffer being allocated if one is not available.
    /// </remarks>
    public static ArrayPool<T> Shared
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get { return Volatile.Read(ref s_sharedInstance) ?? EnsureSharedCreated(); }
    }

ArrayPool 不使用 WeakReference 或任何类似的机制来确保正确处理。如果你从ArrayPool.Shared租用缓冲区，你必须return它，否则会导致内存泄漏。

所以是的，我想说尊重 IDisposable 在这里非常重要。

Answer 2

由 HttpResponseException 包装的响应将由 asp.net 框架处理，就像您 return 从您的操作中得到的任何其他响应一样。您可以通过创建虚拟响应消息轻松地自行测试：

class DummyResponse : HttpResponseMessage {
    public DummyResponse(HttpStatusCode statusCode) : base(statusCode) {
    }

    protected override void Dispose(bool disposing) {
        Console.WriteLine("dispose called");
        base.Dispose(disposing);
    }
}

然后用该响应抛出 HttpResponseException 并将断点放在 Dispose 重写中。您会观察到 Dispose 被调用，如果您查看调用堆栈，您会看到 HttpControllerHandler 负责执行此操作（在 asp.net web api 控制器中）。

请注意，此异常由负责调用您的 api 控制器操作的 ApiControllerActionInvoker、class 捕获。然后它只是抓取 yourException.Response 并通过管道将其向前推，因此抛出此异常与仅 return 来自您的 api 控制器操作的相应响应没有什么不同。应该清楚的是，我认为框架将在处理完所有这些响应后将其处理掉。否则这将是非常糟糕的设计。

所以，不要让那些 RegisterForDispose 使您的代码混乱，让框架为您处理。

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