替换ASP.NET Core 1.0中间件中的响应流

Question

我想在我的ASP.NET Core 1.0项目中编写自定义中间件,它将取代原始框架的Http响应流到我自己的,所以我将能够对它执行读/寻/写操作(原来不可能在原来的2)流)在进一步的代码中,即在动作或过滤器中.

我已经开始使用以下代码:

public class ReplaceStreamMiddleware
{
    protected RequestDelegate NextMiddleware;

    public ReplaceStreamMiddleware(RequestDelegate next)
    {
        NextMiddleware = next;
    }

    public async Task Invoke(HttpContext httpContext)
    {       
        using (var responseStream = new MemoryStream())
        {
            var fullResponse = httpContext.Response.Body;
            httpContext.Response.Body = responseStream;
            await NextMiddleware.Invoke(httpContext);
            responseStream.Seek(0, SeekOrigin.Begin);
            await responseStream.CopyToAsync(fullResponse);
        }   
    }
}

用下面的代码的问题是,有时在fullResponse流已经关闭,在调用的时候await responseStream.CopyToAsync(fullResponse);所以它抛出一个异常,无法访问已关闭的流.

当我在浏览器中加载页面然后在完全加载之前刷新时,很容易观察到这种奇怪的行为.

我想知道:

1. 为什么会这样？
2. 怎么预防呢？
3. 我的解决方案是一个好主意还是有另一种方法来取代响应流？

Answer 1

例外不是来自你的CopyToAsync.它来自您的一个代码的调用者:

您没有恢复原始响应流HttpContext.因此,无论谁调用您的中间件都会收回关闭MemoryStream.

这是一些有效的代码:

app.Use(async (httpContext, next) =>
{
    using (var memoryResponse = new MemoryStream())
    {
        var originalResponse = httpContext.Response.Body;
        try
        {
            httpContext.Response.Body = memoryResponse;

            await next.Invoke();

            memoryResponse.Seek(0, SeekOrigin.Begin);
            await memoryResponse.CopyToAsync(originalResponse);
        }
        finally
        {
            // This is what you're missing
            httpContext.Response.Body = originalResponse;
        }
    }
});

app.Run(async (context) =>
{
    context.Response.ContentType = "text/other";
    await context.Response.WriteAsync("Hello World!");
});