Get exception in Middleware

Question

I want to create an AspNetCore Middleware (old IHttpModule), which should capture exceptions (later on save them or something similar)

However, i don't know how to capture the Exception inside Middleware, although the HttpStatusCode is 500

Here is what i have:

// Middleware
internal class ExceptionMiddleware
{
    private readonly RequestDelegate _next;

    public ExceptionMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        try
        {
            await _next.Invoke(context);
        }
        catch (Exception ex)
        {
            // never gets here
        }

        // 500
        HttpStatusCode statusCode = (HttpStatusCode)context.Response.StatusCode;

        // how to get error message?
    }
}

// Middleware Extension method
public static class ExceptionMiddlewareExtensions
{
    public static IApplicationBuilder UseExceptionMiddleware(this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<ExceptionMiddleware>();
    }
}

// Startup.cs
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    app.UseExceptionMiddleware();

    loggerFactory.AddConsole(Configuration.GetSection("Logging"));
    loggerFactory.AddDebug();

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseBrowserLink();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
    }

    app.UseStaticFiles();

    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });
}

I trigger the exception like this:

public IActionResult Index()
{
    var test = 0;
    var a = 100 / test;

    return View();
}

Answer 1

You correctly use try-catch in middleware.

But your problem is that you also register the ExceptionHandler middleware (app.UseExceptionHandler). This middleware catches all unhandled exceptions and set 500 status code if could handle it.

---

As a possible solution consider to swap the order of middlewares, so your middleware will be the first one that catches the exceptions occurred further in the pipeline:

app.UseExceptionHandler("/Home/Error");
app.UseExceptionMiddleware();