The easiest to understand definition of Aggregate is
that it performs an operation on each element of the list taking into account the operations that have gone before. That is to say it performs the action on the first and second element and carries the result forward. Then it operates on the previous result
and the third element and carries forward. etc.
Example 1. Summing numbers
var nums = new[]{1,2,3,4};
var sum = nums.Aggregate( (a,b) => a + b);
Console.WriteLine(sum); // output: 10 (1+2+3+4)
This adds 1 and 2 to
make 3.
Then adds 3 (result
of previous) and 3 (next
element in sequence) to make 6.
Then adds 6 and 4 to
make 10.
Example 2. create a csv from an array of strings
var chars = new []{"a","b","c", "d"};
var csv = chars.Aggregate( (a,b) => a + ',' + b);
Console.WriteLine(csv); // Output a,b,c,d
This works in much the same way. Concatenate a a
comma and b to
make a,b.
Then concatenatesa,b with
a comma and c to
make a,b,c.
and so on.
Example 3. Multiplying numbers using a seed
For completeness, there is an overload of Aggregate which
takes a seed value.
var multipliers = new []{10,20,30,40};
var multiplied = multipliers.Aggregate(5, (a,b) => a * b);
Console.WriteLine(multiplied); //Output 1200000 ((((5*10)*20)*30)*40)
Much like the above examples, this starts with a value of 5 and
multiplies it by the first element of the sequence 10 giving
a result of 50.
This result is carried forward and multiplied by the next number in the sequence 20 to
give a result of 1000.
This continues through the remaining 2 element of the sequence.
Live examples: http://rextester.com/ZXZ64749
Docs: http://msdn.microsoft.com/en-us/library/bb548651.aspx