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foreach, in (C# Reference)

Visual Studio 2008

The foreach statement repeats a group of embedded statements for each element in an array or an object collection that implements the System.Collections.IEnumerable or System.Collections.Generic.IEnumerable<T> interface. The foreach statement is used to iterate through the collection to get the information that you want, but can not be used to add or remove items from the source collection to avoid unpredictable side effects. If you need to add or remove items from the source collection, use a for loop.

The embedded statements continue to execute for each element in the array or collection. After the iteration has been completed for all the elements in the collection, control is transferred to the next statement following the foreach block.

At any point within the foreach block, you can break out of the loop by using the break keyword, or step to the next iteration in the loop by using the continue keyword.

A foreach loop can also be exited by the goto, return, or throwstatements.

For more information about the foreach keyword and code samples, see the following topics:

Using foreach with Arrays (C# Programming Guide)

How to: Access a Collection Class with foreach (C# Programming Guide)

In this example, foreach is used to display the contents of an array of integers.

C#
Copy 
class ForEachTest
{
    static void Main(string[] args)
    {
        int[] fibarray = new int[] { 0, 1, 2, 3, 5, 8, 13 };
        foreach (int i in fibarray)
        {
            System.Console.WriteLine(i);
        }
    }
}
/*
Output:
0
1
2
3
5
8
13
*/

For more information, see the following sections in the C# Language Specification:

  • 5.3.3.16 Foreach statements

  • 8.8.4 The foreach statement

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Queuing element removal in foreach block
Instead of falling back to ordinary for loop, you can queue the removal of the elements within the foreach block.

var queue = new Queue<Action>();
foreach (var element in collection) {
    var e = element;  // We don't want the reference to element to be updated along the iterations.
    if (e == something)  // Our removal condition
        queue.Enqueue(() => collection.Remove(e));
}
foreach (var action in queue)
    action();  // Here we can safely remove the elements as desired.

This code is, however, not efficient for larger collections.

        

        

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              foreach will throw an exception if enumeration is null 
                
        
 
        

            
If the enumeration you are looping over is null, foreach will throw an exception. Thus you may need an if-test to check for a null enumeration before doing your foreach loop.



    if (null != fibarray) // if you are not certain that fibarray is allocated...
    {
        foreach (int i in fibarray)
        {
            System.Console.WriteLine(i);
        }
    }

        

        

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