C# Language Tutorial => Build your own Linq operators for...

Example

One of the great things about Linq is that it is so easy to extend. You just need to create an extension method whose argument is IEnumerable<T>.

public namespace MyNamespace
{
    public static class LinqExtensions
    {
        public static IEnumerable<List<T>> Batch<T>(this IEnumerable<T> source, int batchSize)
        {
            var batch = new List<T>();
            foreach (T item in source)
            {
                batch.Add(item);
                if (batch.Count == batchSize)
                {
                    yield return batch;
                    batch = new List<T>();
                }
            }
            if (batch.Count > 0)
                yield return batch;
        }
    }
}

This example splits the items in an IEnumerable<T> into lists of a fixed size, the last list containing the remainder of the items. Notice how the object to which the extension method is applied is passed in (argument source) as the initial argument using the this keyword. Then the yield keyword is used to output the next item in the output IEnumerable<T> before continuing with execution from that point (see yield keyword).

This example would be used in your code like this:

//using MyNamespace;
var items = new List<int> { 2, 3, 4, 5, 6 };
foreach (List<int> sublist in items.Batch(3))
{
    // do something
}

On the first loop, sublist would be {2, 3, 4} and on the second {5, 6}.

Custom LinQ methods can be combined with standard LinQ methods too. e.g.:

//using MyNamespace;
var result = Enumerable.Range(0, 13)         // generate a list
                       .Where(x => x%2 == 0) // filter the list or do something other
                       .Batch(3)             // call our extension method
                       .ToList()             // call other standard methods

This query will return even numbers grouped in batches with a size of 3: {0, 2, 4}, {6, 8, 10}, {12}

Remember you need a using MyNamespace; line in order to be able to access the extension method.

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