Why MemoryStream() has data even though it is not initialized with any source of data-CodePudding

I have a test .Net Core application and I want to read http responses from pipeline for my personal practice.

This minimum code example provides http response. I know it is destroying the response body which I need to fix it latter, but it is not my question. My question is how does MemoryStream() has http response inside it even though I didn't initialized it with any source of data?

"Startup.cs":

public void Configure(IApplicationBuilder app)
{
    app.Use(async (context, next) =>
    {
        using (var swapStream = new MemoryStream())
        {
            context.Response.Body = swapStream;

            await next.Invoke();

            swapStream.Seek(0, SeekOrigin.Begin);
            string responseBody = new StreamReader(swapStream).ReadToEnd();
        }
    });
 }

As you can see MemoryStream() is not initialized with any source of data at all. But in the end in responseBody I can see my http response. What makes me more confused is that if I remove context.Response.Body = swapStream; then I don't have http response in responseBody.

It would be appreciated if someone could help me to understand how does this code works.

CodePudding user response：

Something else is writing to the stream, it happens when you call

await next.Invoke();

You are replacing the response stream with your memorystream, and something later in the chain writes to that stream.