Rx framework – IObservable<T>.Merge

You can become a serverless blackbelt. Enrol in my course Learn you some Lambda best practice for great good! and learn best practices for performance, cost, security, resilience, observability and scalability. By the end of this course, you should be able to make informed decisions on which AWS service to use with Lambda and how to build highly scalable, resilient and cost-efficient serverless applications.

In my previous post I used the IObservable<T>.Zip extension method in the Drag-and-Drop example, which takes two observable collections and runs a function over them to return a new observable collection of potentially a different type.

But what if you just want to simply merge the two observable collections into one stream? Well, Rx API has another extension method called Merge. To use it you can either invoke it as extension method:

var zs = xs.Merge(ys);

or you can invoke it as static method on more than two observable collections:

var zs = Observable.Merge(xs, ys, us, vs);

To use the Merge method, all the observable collections being merged need to be of the same type.

There are two things you should keep in mind:

1. OnCompleted is fired on the composite observable collection (zs) at the point the last OnCompleted is fired on any of the merged observable collections:

image

2. OnError is fired on the composite observable collection (zs) at the point the first OnError is fired on any of the merged observable collections, any subsequent values on the merged observable collection will not make it onto the composite collection:

image

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter


Hi, I’m Yan. I’m an AWS Serverless Hero and the author of Production-Ready Serverless.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.


Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.


Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!


Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!


Are you working with Serverless and looking for expert training to level-up your skills? Or are you looking for a solid foundation to start from? Look no further, register for my Production-Ready Serverless workshop to learn how to build production-grade Serverless applications!

Find a workshop near you