Using Aggregate in LINQ

The System.Linq namespace has a load of useful extension methods like Where, Select etc. etc. that allow us to chain up bits of code that operate over sequences of data, allowing us to apply functional-style programming to our data.

There is one method which is often overlooked yet it is probably the one that lends itself best to functional programming is the Aggregate() method. Unlike methods such as a Select, which, given a set of n items, projects a set of n other items, Aggregate can be used as a way of merging a collection of items into a different number of items. Indeed, some LINQ methods can be implemented easily with aggregate, such as Sum: –



The syntax looks a bit bizarre, especially when you look at the function signature of the method (including the overloads), but essentially the method takes in a function which itself takes in two values and returns another: –

  • accumulator, which is an arbitrary object which is passed through every item in the collection. In the default overload of Aggregate, this is the same type as the source collection e.g. Int32.
  • value, which is the next value in the chain e.g. 1, then 2, then 3 etc.
  • result, which will become the accumulator in the next iteration

So, if we were to expand the above bit of code with debugging statements etc., it would look something like this: –image

Note that with the default function overload, the initial value of the accumulator is the first value in the collection (1), aka the “seed” value.

More complex uses of Aggregate

Let’s say we wanted to print a single string out which is all of the numbers separated by a space e.g. “1 2 3 4 5 6 7 8 9 10”. Common LINQ methods wouldn’t be appropriate for this. You could use Select to get the string representation, but would get a sequence of 10 strings rather than a single one. You might now fall back to foreach loops etc., but this is where Aggregate is useful: –


This overload of Aggregate takes in two arguments – the first is a “seed value” which will be the initial value of the accumulator, in our case an empty String. Every iteration takes the accumulator, appends the next number to it and returns the resultant String as the next accumulator, which gives us the following (debug statements added): –


Simples! (obviously in a real world example you might use a StringBuilder as your Accumulator instead).

Notice how in this example we didn’t return the same type as the collection that we operated over (i.e. Int32). We can use this technique to do all sorts of funky things to collections that you might not have considered before.


Aggregate is a rarely used but extremely powerful LINQ method. In my next post, I’ll build on this showing some more powerful (and perhaps useful!) examples of Aggregate.


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