C# DotNet Core Middleware Wrap Response

Question

I have a simple controller action which looks like:

    public Task<IEnumerable<Data>> GetData()
    {
        IEnumerable<Data> data = new List<Data>();
        return data;
    }

I want to be able to inspect the return value from within the middleware so the JSON would look something like

{
  "data": [
  ],
  "apiVersion": "1.2",
  "otherInfoHere": "here"
}

So my payload always is within data. I know I can do this at a controller level but I don't wan to have to do it on every single action. I would rather do it in middleware once for all.

Here is an example of my middleware:

public class NormalResponseWrapper
{
    private readonly RequestDelegate next;

    public NormalResponseWrapper(RequestDelegate next)
    {
        this.next = next;
    }

    public async Task Invoke(HttpContext context)
    {                
        var obj = context;
        // DO something to get return value from obj
        // Create payload and set data to return value

        await context.Response.WriteAsync(/*RETURN NEW PAYLOAD HERE*/);
    }

Any ideas?

Got the value now but it's to late to return it

        try
        {
            using (var memStream = new MemoryStream())
            {
                context.Response.Body = memStream;
                await next(context);
                memStream.Position = 0;
                object responseBody = new StreamReader(memStream).ReadToEnd();
                memStream.Position = 0;
                await memStream.CopyToAsync(originalBody);
                // By now it is to late, above line sets the value that is going to be returned
                await context.Response.WriteAsync(new BaseResponse() { data = responseBody }.toJson());
            }

        }
        finally
        {
            context.Response.Body = originalBody;
        }

Answer 1

In .NET Core 3.1 or .NET 5

1. Create your response envelope object. Example:

   internal class ResponseEnvelope<T>
   {
     public T Data { set; get; }
     public string ApiVersion { set; get; }
     public string OtherInfoHere { set; get; }
   }
   

2. Derive a class from ObjectResultExecutor

   internal class ResponseEnvelopeResultExecutor : ObjectResultExecutor
   {
    public ResponseEnvelopeResultExecutor(OutputFormatterSelector formatterSelector, IHttpResponseStreamWriterFactory writerFactory, ILoggerFactory loggerFactory, IOptions<MvcOptions> mvcOptions) : base(formatterSelector, writerFactory, loggerFactory, mvcOptions)
    {
    }
   
    public override Task ExecuteAsync(ActionContext context, ObjectResult result)
    {
        var response = new ResponseEnvelope<object>();
        response.Data = result.Value;
        response.ApiVersion = "v1";
        response.OtherInfoHere = "OtherInfo";
   
        TypeCode typeCode = Type.GetTypeCode(result.Value.GetType());
        if (typeCode == TypeCode.Object)
             result.Value = response;
   
        return base.ExecuteAsync(context, result);
     }
   }
   

3. Inject into the DI like

   public void ConfigureServices(IServiceCollection services)
    {
        services.AddSingleton<IActionResultExecutor<ObjectResult>, ResponseEnvelopeResultExecutor>();
   

And the responses should have an envelope. This does not work with primitive types.

Answer 2

Review the comments to get an understanding of what you can do to wrap the response.

public async Task Invoke(HttpContext context) {
    //Hold on to original body for downstream calls
    Stream originalBody = context.Response.Body;
    try {
        string responseBody = null;
        using (var memStream = new MemoryStream()) {
            //Replace stream for upstream calls.
            context.Response.Body = memStream;
            //continue up the pipeline
            await next(context);
            //back from upstream call.
            //memory stream now hold the response data
            //reset position to read data stored in response stream
            memStream.Position = 0;
            responseBody = new StreamReader(memStream).ReadToEnd();
        }//dispose of previous memory stream.
        //lets convert responseBody to something we can use
        var data = JsonConvert.DeserializeObject(responseBody);
        //create your wrapper response and convert to JSON
        var json = new BaseResponse() { 
            data = data, 
            apiVersion = "1.2",
            otherInfoHere = "here"
        }.toJson();
        //convert json to a stream
        var buffer = Encoding.UTF8.GetBytes(json);
        using(var output = new MemoryStream(buffer)) {
            await output.CopyToAsync(originalBody);
        }//dispose of output stream
    } finally {
        //and finally, reset the stream for downstream calls
        context.Response.Body = originalBody;
    }
}