C# LINQ Introduction

Filtering data - A first approach

Ok, now that you know what enumerables are, we can go on and work with them.

In many scenarios you have to filter a data set.

• The data set is a collection of objects (the IEnumerable<T>),
• a decision predicate is an expression or function that can decide if you keep an item.
• You enumerate over the items in the collection and check the predicate.
• If the predicate matches, you keep the item. If not, you trhow it away.

Here is an example: From a list of numbers keep only positive values.

A first approch to filter the data would be to create a new empty list and then enumerate over the numbers and add the positive values to the new list.

The code explained

We successfully created a filter method KeepPositiveNumbers where we can pass a list of arbitrary numbers as input and only get out a list of positive numbers.

The method accepts any IEnumerable<int> and also returns an IEnumerable<int>, so the method works with Arrays, Lists or other data structures that realize the IEnumerable<T> interface.

var numbers1 = KeepPositiveNumbers(new[] { -5, 4, 10, 0, -9 });
var numbers2 = KeepPositiveNumbers(new List<int> { -5, 4, 10, 0, -9 });

It works, we are done

Now time passes by and you have another requirement, that you want to keep only negative numbers.

You have to implement a new filter method for that. 😠😠😠

Or is there a more flexible way? What if we could pass our expression to the filter method in some way?