为 ASP.NET Core 响应正文添加前缀文本

Question

我正在尝试将字符串 )]}',\n 添加到任何 JSON 响应正文中。我认为IAsyncResultFilter 将是我需要使用的，但我没有运气。如果我使用下面的代码，它会将文本追加到响应中，因为调用await next() 会写入响应管道。但是，如果我在此之前尝试查看上下文，我无法判断响应实际上是什么来知道它是否是 JSON。

public class JsonPrefixFilter : IAsyncResultFilter
{
    public async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
    {
        var executed = await next();
        var response = executed.HttpContext.Response;
        if (response.ContentType == null || !response.ContentType.StartsWith("application/json"))
            return;

        var prefix = Encoding.UTF8.GetBytes(")]}',\\n");
        var bytes = new ReadOnlyMemory<byte>(prefix);
        await response.BodyWriter.WriteAsync(bytes);
    }
}

Answer 1

您可以在 Steam 上使用 Seek 来倒带它。问题是，您只能继续添加到默认HttpResponseStream，它不支持搜索。 因此，您可以使用 this SO answer 的技术并暂时将其替换为 MemoryStream：

private Stream ReplaceBody(HttpResponse response)
{
    var originBody = response.Body;
    response.Body = new MemoryStream();
    return originBody;
}
private async Task ReturnBodyAsync(HttpResponse response, Stream originalBody)
{
    response.Body.Seek(0, SeekOrigin.Begin);
    await response.Body.CopyToAsync(originalBody);
    response.Body = originalBody;
}
public async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
{
    var originalBody = ReplaceBody(context.HttpContext.Response); // replace the default stream with MemoryStream

    await next(); // we probably dont care about the return of this call. it's all in the context
    var response = context.HttpContext.Response;
    if (response.ContentType == null || !response.ContentType.StartsWith("application/json"))
        return;

    var prefix = Encoding.UTF8.GetBytes(")]}',\\n");
    var bytes = new ReadOnlyMemory<byte>(prefix);
    response.Body.Seek(0, SeekOrigin.Begin); // now you can seek. but you will notice that it overwrites the response so you might need to make extra space in the buffer
    await response.BodyWriter.WriteAsync(bytes);

    await ReturnBodyAsync(context.HttpContext.Response, originalBody); // revert the reference, copy data into default stream and return it
}

由于您需要恢复对原始流的引用，这使情况变得更加复杂，因此您必须小心。

This SO answer 有更多上下文。

Answer 2

感谢 timur 的帖子，我能够想出这个可行的解决方案。

public class JsonPrefixFilter : IAsyncResultFilter
{
    public async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
    {
        var response = context.HttpContext.Response;

        // ASP.NET Core will always send the contents of the original Body stream back to the client.
        var originalBody = response.Body;

        // We want to write into a memory stream instead of the actual response body for now.
        var ms = new MemoryStream();
        response.Body = ms;

        // After this call the body is written into the memory stream and the properties
        // of the response object are populated.
        await next();

        if (response.ContentType != null && response.ContentType.StartsWith("application/json")) {
            var prefix = Encoding.UTF8.GetBytes(")]}',\\n");

            var prefixMemoryStream = new MemoryStream();
            await prefixMemoryStream.WriteAsync(prefix);
            await prefixMemoryStream.WriteAsync(ms.ToArray());
            prefixMemoryStream.Seek(0, SeekOrigin.Begin);

            // Now put the stream back that .NET wants to use and copy the memory stream to it.
            response.Body = originalBody;
            await prefixMemoryStream.CopyToAsync(response.Body);
        } else {
            // If it's not JSON, don't muck with the stream, so just put things back.
            response.Body = originalBody;
            ms.Seek(0, SeekOrigin.Begin);
            await ms.CopyToAsync(response.Body);
        }
    }
}