`Array.ForEach`

This C# method loops over every element. It calls another method on each element in an array. ForEach() is a declarative syntax form—this simplifies certain code patterns.

Usually the Array.ForEach method is mostly used on arrays of objects. Each object has a method we want to invoke, and we use a lambda to call that method.

`Class` example

Here we create an array of class instances with an initializer expression. Each Dog object has an Add() method that we want to call.

Start We create an array of Dog instances, and set the Size of each dog to a certain integer.

Next The ForEach method calls the lambda (second argument) on each Dog instance we created. Each Dog's size is incremented.

using System;

class Dog
{
    public int Size { get; set; }
    public void Add()
    {
        this.Size++;
    }
}

class Program
{
    static void Main()
    {
        // Create array of objects.
        var dogs = new Dog[]{ new Dog(){ Size = 1 }, new Dog(){ Size = 3 } };

        // Call method on each object.
        Array.ForEach(dogs, dog => dog.Add());
        Console.WriteLine("DONE");
    }
}
DONE

`Console` example

Next we call Array.ForEach on an int array. We cannot modify values within the array, but we can act upon them or pass them to a Console static method.

Info The Action type (the second argument in ForEach) returns no value: it is void. It cannot directly modify elements.

using System;

int[] items = { 10, 100, 1000 };

// Display elements with ForEach.
Array.ForEach(items, element => Console.WriteLine("Element is " + element));Element is 10
Element is 100
Element is 1000

Jagged array example

ForEach uses an Action type as its second argument. We can modify elements in nested jagged arrays with Array.ForEach.

Here This example program shows how to allocate a jagged array and then apply the Array.ForEach method with 2 arguments.

Note The arguments are the array itself, and a lambda expression that represents an Action type instance.

Note 2 The left part before the arrow is the argument name, and the right part is the expression taken upon the argument.

Info The first array element in each subarray is modified. Usually, with ForEach, this style of code is not helpful.

using System;

// Allocate a jagged array and put 3 subarrays into it.
int[][] array = new int[3][];
array[0] = new int[2];
array[1] = new int[3];
array[2] = new int[4];
// Use ForEach to modify each subarray's first element.
// ... Because the closure variable is an array reference, you can change it.
Array.ForEach(array, subarray => subarray[0]++);
foreach (int[] subarray in array)
{
    foreach (int i in subarray)
    {
        Console.Write(i);
    }
    Console.WriteLine();
}
// Apply the same routine with ForEach again.
Array.ForEach(array, subarray => subarray[0]++);
foreach (int[] subarray in array)
{
    foreach (int i in subarray)
    {
        Console.Write(i);
    }
    Console.WriteLine();
}10
100
1000
20
200
2000

Discussion

The Action delegate receives a formal parameter list. These arguments are copied by value. When we copy an element of an array as a parameter, it becomes a scalar variable.

So With ForEach, we cannot directly modify array elements. The change is not reflected in the array.

Thus Array.ForEach is not a "map" method. It is a loop abstraction, but not a good way to mutate every array element.

Also A LINQ query, with the ToArray method, is an easier way to create a modified array.

Summary

Array.ForEach is a declarative loop. By applying this method, we can invoke a method on each array element. Or we can modify something based on a reference (class) element.

Array.ForEach