foreach & IEnumerable

Syntax

foreach (Type item in collection) {\n  // code\n}

Examples

List<string> names = new List<string> { "Fola", "Zain", "Jamal" };

foreach (string name in names)
{
    Console.WriteLine(name);
}

int[] numbers = { 1, 2, 3 };
foreach (int n in numbers)
{
    Console.WriteLine(n * n);
}

Dictionary<string, int> scores = new Dictionary<string, int>
{
    ["Alice"] = 85,
    ["Bob"] = 92
};

foreach (var entry in scores)  // type inferred as KeyValuePair<string, int>
{
    Console.WriteLine($"{entry.Key}: {entry.Value}");
}

public class NumberRange : IEnumerable<int>
{
    private int _start, _end;
    public NumberRange(int start, int end) { _start = start; _end = end; }

    public IEnumerator<int> GetEnumerator()
    {
        for (int i = _start; i <= _end; i++)
            yield return i;
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        => GetEnumerator();
}

foreach (int n in new NumberRange(1, 5))
{
    Console.WriteLine(n);  // 1, 2, 3, 4, 5
}

Best practices

• Use foreach by default for reading through a collection - it is safer and more readable than a manual indexed loop
• Don't modify a collection's structure (adding/removing items) while iterating over it with foreach - it throws an InvalidOperationException
• Implement IEnumerable<T> on a custom class when it genuinely represents a collection of things, so it works naturally with foreach and LINQ
• Use yield return inside GetEnumerator() (or any iterator method) for a concise way to implement custom iteration logic

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation