Foreach Loops in

About Foreach Loops

enumerables can be iterated over using a foreach loop:

char[] vowels = new [] { 'a', 'e', 'i', 'o', 'u' };

foreach (char vowel in vowels)
{
    // Output the vowel
    System.Console.Write(vowel);
}

// => aeiou

A foreach loop consists of three parts:

1. The declaration: define the variable that will contain the iterated value. This variable can (only) be used inside the loop.
2. The iterable object: the object that can be iterated over.
3. The loop body.

for vs foreach loops

However, generally a foreach loop is preferrable over a for loop for the following reasons:

• A foreach loop is guaranteed to iterate over all values. With a for loop, it is easy to miss elements, for example due to an off-by-one error.
• A foreach loop is more declarative, your code is communicating what you want it to do, instead of a for loop that communicates how you want to do it.
• A foreach loop is foolproof, whereas with for loops it is easy to have an off-by-one error.
• A foreach loop works on all collection types, including those that don't support using an indexer to access elements.

To guarantee that a foreach loop will iterate over all values, the compiler will not allow updating of a collection within a foreach loop:

char[] vowels = new [] { 'a', 'e', 'i', 'o', 'u' };

foreach (char vowel in vowels)
{
    // This would result in a compiler error
    // vowel = 'Y';
}

A for loop does have some advantages over a foreach loop:

• You can start or stop at the index you want.
• You can use any (Boolean) termination condition you want.
• You can skip elements by customizing the incrementing of the loop variable.
• You can process collections from back to front by counting down.
• You can use for loops in scenarios that don't involve collections.

Related Topics:

• You should be aware that C# supports multi-dimensional arrays like int[,] arr = new int[10, 5] which can be very useful.
• You should also be aware that you can instantiate objects of type System.Array with Array.CreateInstance. Such objects are of little use - mainly for interop with VB.NET code. They are not interchangeable with standard arrays (T[]). They can have a non-zero lower bound.

Both the above topics are discussed more fully in a later exercise.