The foreach statement is a convenient way to iterate over the elements of an array. It can also enumerate the elements of a collection, provided that the collection class has implemented the System.Collections.IEnumerator and System.Collections.IEnumerable interfaces.
Example1.
The following code sample illustrates how to write a collection class that can be used with foreach. The class is a string tokenizer, similar to the C run-time function strtok.
Notice that, internally, Tokens uses an array, which implements IEnumerator and IEnumerable itself. The code sample could have leveraged the array’s enumeration methods as its own, but that would have defeated the purpose of this example.
In C#, it is not strictly necessary for a collection class to inherit from IEnumerable and IEnumerator in order to be compatible with foreach; as long as the class has the required GetEnumerator, MoveNext, Reset, and Current members, it will work with foreach. Omitting the interfaces has the advantage of allowing you to define the return type of Current to be more specific than object, thereby providing type-safety.
// tokens.cs
using System;
// The System.Collections namespace is made available:
using System.Collections;
// Declare the Tokens class:
public class Tokens : IEnumerable
{
private string[] elements;
Tokens(string source, char[] delimiters)
{
// Parse the string into tokens:
elements = source.Split(delimiters);
}
// IEnumerable Interface Implementation:
// Declaration of the GetEnumerator() method
// required by IEnumerable
public IEnumerator GetEnumerator()
{
return new TokenEnumerator(this);
}
// Inner class implements IEnumerator interface:
private class TokenEnumerator : IEnumerator
{
private int position = -1;
private Tokens t;
public TokenEnumerator(Tokens t)
{
this.t = t;
}
// Declare the MoveNext method required by IEnumerator:
public bool MoveNext()
{
if (position < t.elements.Length - 1)
{
position++;
return true;
}
else
{
return false;
}
}
// Declare the Reset method required by IEnumerator:
public void Reset()
{
position = -1;
}
// Declare the Current property required by IEnumerator:
public object Current
{
get
{
return t.elements[position];
}
}
}
// Test Tokens, TokenEnumerator
static void Main()
{
// Testing Tokens by breaking the string into tokens:
// the following code is used to Tokens by breaking "This is a well-done program." into tokens (using ' ' and '-' as separators) and enumerating those tokens with the foreach statement:
Tokens f = new Tokens("This is a well-done program.",
new char[] { ' ', '-' });
foreach (string item in f)
{
Console.WriteLine(item);
}
}
}
Example2.
This sample is equivalent in function to Example 1, but it provides additional type-safety in C# while maintaining interoperability with other languages.
// tokens2.cs
using System;
using System.Collections;
public class Tokens : IEnumerable
{
private string[] elements;
Tokens(string source, char[] delimiters)
{
elements = source.Split(delimiters);
}
// IEnumerable Interface Implementation:
public TokenEnumerator GetEnumerator() // non-IEnumerable version
{
return new TokenEnumerator(this);
}
IEnumerator IEnumerable.GetEnumerator() // IEnumerable version
{
return (IEnumerator)new TokenEnumerator(this);
}
// Inner class implements IEnumerator interface:
public class TokenEnumerator : IEnumerator
{
private int position = -1;
private Tokens t;
public TokenEnumerator(Tokens t)
{
this.t = t;
}
public bool MoveNext()
{
if (position < t.elements.Length - 1)
{
position++;
return true;
}
else
{
return false;
}
}
public void Reset()
{
position = -1;
}
public string Current // non-IEnumerator version: type-safe
{
get
{
return t.elements[position];
}
}
object IEnumerator.Current // IEnumerator version: returns object
{
get
{
return t.elements[position];
}
}
}
// Test Tokens, TokenEnumerator
static void Main()
{
Tokens f = new Tokens("This is a well-done program.",
new char[] { ' ', '-' });
foreach (string item in f) // try changing string to int
{
Console.WriteLine(item);
}
}
}
example1 and example2 Output :
This is a well done program.
- from msdn.com