How do foreach loops work in C#? [closed]

Question

Which types of classes can use foreach loops?

Answer 1

Actually, strictly speaking, all you need to use foreach is a public GetEnumerator() method that returns something with a bool MoveNext() method and a ? Current {get;} property. However, the most common meaning of this is "something that implements IEnumerable/IEnumerable<T>, returning an IEnumerator/IEnumerator<T>.

By implication, this includes anything that implements ICollection/ICollection<T>, such as anything like Collection<T>, List<T>, arrays (T[]), etc. So any standard "collection of data" will generally support foreach.

For proof of the first point, the following works just fine:

using System;
class Foo {
    public int Current { get; private set; }
    private int step;
    public bool MoveNext() {
        if (step >= 5) return false;
        Current = step++;
        return true;
    }
}
class Bar {
    public Foo GetEnumerator() { return new Foo(); }
}
static class Program {
    static void Main() {
        Bar bar = new Bar();
        foreach (int item in bar) {
            Console.WriteLine(item);
        }
    }
}

How does it work?

A foreach loop like foreach(int i in obj) {...} kinda equates to:

var tmp = obj.GetEnumerator();
int i; // up to C# 4.0
while(tmp.MoveNext()) {
    int i; // C# 5.0
    i = tmp.Current;
    {...} // your code
}

However, there are variations. For example, if the enumerator (tmp) supports IDisposable, it is used too (similar to using).

Note the difference in the placement of the declaration "int i" inside (C# 5.0) vs. outside (up C# 4.0) the loop. It's important if you use i in an anonymous method/lambda inside your code-block. But that is another story ;-p

Answer 2

From MSDN:

The foreach statement repeats a group of embedded statements for each element in an array or an object collection. The foreach statement is used to iterate through the collection to get the desired information, but should not be used to change the contents of the collection to avoid unpredictable side effects. (emphasis mine)

So, if you have an array, you could use the foreach statement to iterate through the array, like so:

 int[] fibarray = new int[] { 0, 1, 2, 3, 5, 8, 13 };
    foreach (int i in fibarray)
    {
        System.Console.WriteLine(i);
    }

You could also use it to iterate through a List<T> collection, like so:

List<string> list = new List<string>();

foreach (string item in list)
{
    Console.WriteLine(item);
}

Answer 3

According to the blog post Duck Notation, duck typing is used.