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by Jiří {x2} Činčura

Aggregating with increments

5 Oct 2010 .NET, C#, LINQ

Similarly to previous method for Interleaving two IEnumerables I needed one method for doing kind of aggregate, but also to know about the intermediate steps. I could abuse the Aggregate method either directly or indirectly but I’m always more happy with clean solution.

The method is pretty simple (but before I did some rework, it was ugly 😉). It takes, apart from the IEnumerable, two functions. One for setting up the initial value for the first item and one for getting result for next step.

internal static IEnumerable<TResult> IncrementalAggregate<TSource, TResult>(this IEnumerable<TSource> data,
	Func<TSource, TResult> init,
	Func<TSource, TResult, TResult> nextResult)
	bool first = true;
	TResult intermediate = default(TResult);
	foreach (var item in data)
		if (first)
			intermediate = init(item);
			first = false;
			intermediate = nextResult(item, intermediate);
		yield return intermediate;

With it you can do i.e. summing the numbers and know what the intermediate sums were. Yes, sounds weird, but you might need it, one day as I did. 😃

Profile Picture Jiří Činčura is an independent developer focusing on data and business layers, language constructs, parallelism and databases. Specifically Entity Framework, asynchronous and parallel programming, cloud and Azure. He's Microsoft Most Valuable Professional and you can read his articles, guides, tips and tricks at