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    <p>Actually, <strong>strictly</strong> speaking, all you need to use <code>foreach</code> is a public <code>GetEnumerator()</code> method that returns something with a <code>bool MoveNext()</code> method and a <code>? Current {get;}</code> property. However, the most <strong>common</strong> meaning of this is "something that implements <code>IEnumerable</code>/<code>IEnumerable&lt;T&gt;</code>, returning an <code>IEnumerator</code>/<code>IEnumerator&lt;T&gt;</code>.</p> <p>By implication, this includes anything that implements <code>ICollection</code>/<code>ICollection&lt;T&gt;</code>, such as anything like <code>Collection&lt;T&gt;</code>, <code>List&lt;T&gt;</code>, arrays (<code>T[]</code>), etc. So any standard "collection of data" will generally support <code>foreach</code>.</p> <p>For proof of the first point, the following works just fine:</p> <pre><code>using System; class Foo { public int Current { get; private set; } private int step; public bool MoveNext() { if (step &gt;= 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); } } } </code></pre> <p><strong>How does it work?</strong></p> <p>A foreach loop like <code>foreach(int i in obj) {...}</code> kinda equates to:</p> <pre><code>var tmp = obj.GetEnumerator(); int i; // up to C# 4.0 while(tmp.MoveNext()) { int i; // C# 5.0 i = tmp.Current; {...} // your code } </code></pre> <p>However, there are variations. For example, it the enumerator (tmp) supports <code>IDisposable</code>, it is used too (similar to <code>using</code>).</p> <p><strong>Note the difference</strong> in the placement of the declaration "<code>int i</code>" <em>inside</em> (C# 5.0) vs. <em>outside</em> (up C# 4.0) the loop. It's important if you use <code>i</code> in an anonymous method/lambda inside your code-block. But that is another story ;-p</p>
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    1. POHow do foreach loops work in C#?
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    1. USPaul Groke
    1. USMarc Gravell
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    1. CO+1 for going in depth. I normally don't go that in depth for a question that may be a 'beginner' question since it would seem overwhelming to the new programmer.
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      1. USGeorge Stocker
    2. COTrue, Gortok - so I followed up with the stuff about lists/arrays/etc.
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      1. USMarc Gravell
    3. COWould be good to mention the implicit runtime casting to the loop variable - can produce type incompatibility exceptions.
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      1. USDaniel Earwicker
 

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