LINQ – choosing between Concat() and Union()

In Linq To Objects, there are two ways you can join two sequences together, using either Concat() or Union(), and as I was wondering how the two differs I came across this post:

The main thing to take away from this article is:

“If you care about the duplicates, Union() is necessary. However, in the case where you can’t have duplicates in the second sequence or you don’t care, Concat() is a better choice.”

Thinking in T-SQL terms:

Concat() = UNION ALL

Union() = UNION

Joining two sequences using Union()

If by some chance you’re looking to join two potentially duplicated lists together, and you don’t want duplicates in the resulting list, see this question on StackOverflow and see Jon Skeet’s answer for a nice and clean way to do this:

Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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