The foreach keyword allows you to loop over items in an enumerable collection. Prior to C# 9 this included arrays, dynamic objects, and objects from classes that implemented the GetEnumerator method or IEnumerable interface.

This feature allows foreach to utilise an GetEnumerator extension method automatically when available.

Code

Now

void PrintAddress()
{
    foreach (var line in address)
        Console.WriteLine(line);
}

static class Extension
{
    public static IEnumerator<string> GetEnumerator(this Address address)
    {
        return new AddressLineEnumerator(address);
    }
}

Before

void PrintAddress()
{
    foreach (var line in new AddressLineEnumeratorAdapter(address))
        Console.WriteLine(line);
}

class AddressLineEnumeratorAdapter : IEnumerable<string>
{
    readonly Address address;

    public AddressLineEnumeratorAdapter(Address address)
    {
        this.address = address;
    }

    public IEnumerator<string> GetEnumerator()
    {
        return new AddressLineEnumerator(address);
    }
}

More information

• Specification
• Make a class usable in foreach