Web API middleware short circuiting unexpectedly

Question

The following code is part of an error handling middleware, whose goal is to provide consistent formatting for the client even if an error is thrown.

I am trying to serialize a response as XML when the Accept header is set to application/xml, otherwise return JSON. This article helped get me started: https://www.devtrends.co.uk/blog/handling-errors-in-asp.net-core-web-api

if (context.Request.Headers["Accept"] == "application/xml")
{
    context.Response.ContentType = "application/xml";

    using (var stringwriter = new StringWriter())
    {
        var serializer = new XmlSerializer(response.GetType());
        serializer.Serialize(stringwriter, response);
        await context.Response.WriteAsync(stringwriter.ToString());
    }
}
else {
    context.Response.ContentType = "application/json";
    var json = JsonConvert.SerializeObject(response);
    await context.Response.WriteAsync(json);
}

The else block works as expected. If I set a breakpoint on the line where the XmlSerializer is declared, execution halts. If I set a breakpoint on the following line, the breakpoint is never hit; a response has already been sent to the client.

My middleware is configured like this:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseStatusCodePagesWithReExecute("/error/{0}");
        app.UseExceptionHandler("/error/500");
        app.UseHsts();
        app.UseMiddleware<ErrorWrappingMiddleware>();
        app.UseMvc();
    }

Why is a response returned to the client before context.Response.WriteAsync(stringwriter.ToString()); is called in the if block?

WebAPi has content-negotiation built in (https://learn.microsoft.com/en-us/aspnet/core/web-api/advanced/formatting?view=aspnetcore-2.2), Is there any reason to write this middleware? — Vivek Natarajan, Jul 19 '19 at 23:51
I've seen this page and added `AddXmlSerializerFormatters()` to my configuration, but when an error is returned (status code 500 for instance) the response body is blank. I want a consistently formatted response body, hence the additional middleware. — McGlothlin, Jul 22 '19 at 14:40
When using XML formatters, return only types don't return IEnumerable of types this will throw exception. — Vivek Natarajan, Jul 23 '19 at 16:19
Can we get a more complete example? Also what happens if you set a try catch around var serializer = new XmlSerializer(response.GetType()); serializer.Serialize(stringwriter, response); await context.Response.WriteAsync(stringwriter.ToString()); — Jonathan Alfaro, Jul 26 '19 at 19:47

score 1 · Accepted Answer · answered Jul 26 '19 at 22:15

I figured out the problem, it was a silly mistake.

To test 500 errors, I was intentionally adding a property to a model that didn't have a corresponding column in the database. This throws a SqlException on an invalid column name. There was however a second exception I missed that told me exactly what the problem was:

ApiResponse cannot be serialized because it does not have a parameterless constructor.

This occurred in the call to serializer.Serialize(stringwriter, response). The example I referenced creates an ApiResponse class (the response I attempted to serialize). However this class does not have a parameterless constructor in the example I was learning from, and I wasn't aware this was necessary until I found this answer. This second exception halted execution, but since I forgot to turn on development mode and I was trying to throw an exception, I didn't notice the second one.

Web API middleware short circuiting unexpectedly

1 Answers1