using AppFunc = Func<IDictionary<string, object>, Task>;

public class OwinExceptionHandlerMiddleware
{
    private readonly AppFunc _next;

    public OwinExceptionHandlerMiddleware(AppFunc next)
    {
        if (next == null)
        {
            throw new ArgumentNullException("next");
        }

        _next = next;
    }

    public async Task Invoke(IDictionary<string, object> environment)
    {
        try
        {
            await _next(environment);
        }
        catch (Exception ex)
        {
            try
            {

                var owinContext = new OwinContext(environment);

                NLogLogger.LogError(ex, owinContext);

                HandleException(ex, owinContext);

                return;
            }
            catch (Exception)
            {
                // If there's a Exception while generating the error page, re-throw the original exception.
            }
            throw;
        }
    }
    private void HandleException(Exception ex, IOwinContext context)
    {
        var request = context.Request;

        //Build a model to represet the error for the client
        var errorDataModel = NLogLogger.BuildErrorDataModel(ex);

        context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
        context.Response.ReasonPhrase = "Internal Server Error";
        context.Response.ContentType = "application/json";
        context.Response.Write(JsonConvert.SerializeObject(errorDataModel));

    }

}

public static class OwinExceptionHandlerMiddlewareAppBuilderExtensions
{
    public static void UseOwinExceptionHandler(this IAppBuilder app)
    {
        app.Use<OwinExceptionHandlerMiddleware>();
    }
}