How to catch an exception and respond with a status code in .NET Core

Question

I am running a .Net Core Web API project. I have a startup file (below). In the Startup.ConfigureServices(...) method, I add a factory method that creates an instance of IFoo. I want to catch any exceptions that the IFooFactory throws and return a better error message with a status code. At the moment I getting a 500 Error with the exception Message. Can anyone help?

public interface IFooFactory
{
    IFoo Create();  
}

public class FooFactory : IFooFactory
{
    IFoo Create()
    {
        throw new Exception("Catch Me!");
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddScoped<IFooFactory,FooFactory>();
        services.AddScoped(serviceProvider => {
            IFooFactory fooFactory = serviceProvider.GetService<IFooFactory>();
            return fooFactory.Create(); // <== Throws Exception
        });
    }
}

Answer 1

So I've posted two different answers as I read the question in two different ways - lots of deleting/undeleting/editing - not sure which one actually answers your question:

To figure out what is going wrong when the app starts and doesn't work at all try this:

Use the developer exception page in Startup :

public void Configure(IApplicationBuilder app, IHostingEnvironment env,
                           ILoggerFactory loggerFactory)
{
    app.UseDeveloperExceptionPage();
}

And in the Program class:

public static void Main(string[] args)
{
    var host = new WebHostBuilder()
        .UseKestrel()
        .UseContentRoot(Directory.GetCurrentDirectory())
        .UseIISIntegration()
        .UseStartup<Startup>()
        .UseApplicationInsights()
        .CaptureStartupErrors(true) // useful for debugging
        .UseSetting("detailedErrors", "true") // what it says on the tin
        .Build();

    host.Run();
}

If you want to handle an occasional exception when the api is generally working then you can use some middleware:

public class ExceptionsMiddleware
{
    private readonly RequestDelegate _next;

    /// <summary>
    /// Handles exceptions
    /// </summary>
    /// <param name="next">The next piece of middleware after this one</param>
    public ExceptionsMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    /// <summary>
    /// The method to run in the piepline
    /// </summary>
    /// <param name="context">The current context</param>
    /// <returns>As task which is running the action</returns>
    public async Task Invoke(HttpContext context)
    {
        try
        {
            await _next.Invoke(context);
        }
        catch(Exception ex)
        {
            // Apply some logic based on the exception
            // Maybe log it as well - you can use DI in
            // the constructor to inject a logging service

            context.Response.StatusCode = //Your choice of code
            await context.Response.WriteAsync("Your message");
        }
    }
}

There is one 'gotcha' with this - You can't write the status code if the response header has already been sent.

You configure the middleware in the Startup class using the Configure method:

public void Configure(IApplicationBuilder app, IHostingEnvironment env,
                           ILoggerFactory loggerFactory)
{
    app.UseMiddleware<ExceptionsMiddleware>();
}