How to use foreach keyword on custom Objects in C#

Question

Can someone share a simple example of using the foreach keyword with custom objects?

Answer 1

Given the tags, I assume you mean in .NET - and I'll choose to talk about C#, as that's what I know about.

The foreach statement (usually) uses IEnumerable and IEnumerator or their generic cousins. A statement of the form:

foreach (Foo element in source) {     // Body } 

where source implements IEnumerable<Foo> is roughly equivalent to:

using (IEnumerator<Foo> iterator = source.GetEnumerator()) {     Foo element;     while (iterator.MoveNext())     {         element = iterator.Current;         // Body     } } 

Note that the IEnumerator<Foo> is disposed at the end, however the statement exits. This is important for iterator blocks.

To implement IEnumerable<T> or IEnumerator<T> yourself, the easiest way is to use an iterator block. Rather than write all the details here, it's probably best to just refer you to chapter 6 of C# in Depth, which is a free download. The whole of chapter 6 is on iterators. I have another couple of articles on my C# in Depth site, too:

• Iterators, iterator blocks and data pipelines
• Iterator block implementation details

As a quick example though:

public IEnumerable<int> EvenNumbers0To10() {     for (int i=0; i <= 10; i += 2)     {         yield return i;     } }  // Later foreach (int x in EvenNumbers0To10()) {     Console.WriteLine(x); // 0, 2, 4, 6, 8, 10 } 

To implement IEnumerable<T> for a type, you can do something like:

public class Foo : IEnumerable<string> {     public IEnumerator<string> GetEnumerator()     {         yield return "x";         yield return "y";     }      // Explicit interface implementation for nongeneric interface     IEnumerator IEnumerable.GetEnumerator()     {         return GetEnumerator(); // Just return the generic version     } } 

Answer 2

(I assume C# here)

If you have a list of custom objects you can just use the foreach in the same way as you do with any other object:

List<MyObject> myObjects = // something foreach(MyObject myObject in myObjects) {      // Do something nifty here } 

If you want to create your own container you can use the yield keyword (from .Net 2.0 and upwards I believe) together with the IEnumerable interface.

class MyContainer : IEnumerable<int> {     private int max = 0;     public MyContainer(int max)     {         this.max = max;     }      public IEnumerator<int> GetEnumerator()     {         for(int i = 0; i < max; ++i)             yield return i;     }      IEnumerator IEnumerable.GetEnumerator()     {         return GetEnumerator();     } } 

And then use it with foreach:

MyContainer myContainer = new MyContainer(10); foreach(int i in myContainer)     Console.WriteLine(i);