The foreach Statement

The foreach Statement

C# provides a foreach statement that is often useful for iterating through the elements of certain data structures. A foreach can be used when all of the following conditions hold:

1. The data structure is a subtype of either IEnumerable or IEnumerable<T> for some type T.
2. You do not need to know the locations in the data structure of the individual elements.
3. You do not need to modify the data structure with this loop.

For example, the string class is a subtype of both IEnumerable and IEnumerable<Char>. To see that this is the case, look in the documentation for the string class. In the “Implements” section, we see all of the interfaces implemented by string. Because string implements both of these interfaces, it is a subtype of each. We can therefore iterate through the elements (i.e., the characters) of a string using a foreach statement, provided we don’t need to know the location of each character in the string (because a string is immutable, we can’t change its contents).

Suppose, for example, that we want to find out how many times the letter ‘i’ occurs in a string s. Because we don’t need to know the locations of these occurrences, we can iterate through the characters with a foreach loop, as follows:

int count = 0;
foreach (char c in s)
{
    if (c == 'i')
    {
        count++;
    }
}

The foreach statement requires three pieces of information:

• The type of the elements in the data structure (char in the above example).
• The name of a new variable (c in the above example). The type of this variable will be the type of the elements in the data structure (i.e., char in the above example). It will take on the values of the elements as the loop iterates.
• Following the keyword in, the data structure being traversed (s in the above example).

The loop then iterates once for each element in the data structure (unless a statement like return or break causes it to terminate prematurely). On each iteration, the variable defined in the foreach statement stores one of the elements in the data structure. Thus, in the above example, c takes the value of a different character in s on each iteration. Note, however, that we have no access to the location containing c on any particular iteration - this is why we don’t use a foreach loop when we need to know the locations of the elements. Because c takes on the value of each character in s, we are able to count how many of them equal ‘i’.

Occasionally, it may not be obvious what type to use for the foreach loop variable. In such cases, if the data structure is a subtype of IEnumerable<T>, then the type should be whatever type is used for T. Otherwise, it is safe to use object. Note, however, that if the data structure is not a subtype of IEnumerable<T>, but you know that the elements are some specific subtype of object, you can use that type for the loop variable - the type will not be checked until the code is executed. For example, ListBox is a class that implements a GUI control displaying a list of elements. The elements in the ListBox are accessed via its Items property, which gets a data structure of type ListBox.ObjectCollection. Any object can be added to this data structure, but we often just add strings. ListBox.ObjectCollection is a subtype of IEnumerable; however, it is permissible to set up a foreach loop as follows:

foreach (string s in uxList.Items)
{

}

where uxList is a ListBox variable. As long as all of the elements in uxList.Items are strings, no exception will be thrown.

While the foreach statement provides a clean way to iterate through a data structure, it is important to keep in mind its limitations. First, it can’t even be used on data structures that aren’t subtypes of IEnumerable or IEnumerable<T>. Second, there are many cases, especially when iterating through arrays, where the processing we need to do on the elements depends on the locations of the elements. For example, consider the problem of determining whether two arrays contain the same elements in the same order. For each element of one array, we need to know if the element at the same location in the other array is the same. Because the locations are important, a foreach loop isn’t appropriate - we should use a for loop instead. Finally, a foreach should never be used to modify a data structure, as this causes unpredictable results.

Even when a foreach would work, it is not always the best choice. For example, in order to determine whether a data structure contains a given element, we could iterate through the structure with a foreach loop and compare each element to the given element. While this would work, most data structures provide methods for determining whether they contain a given element. These methods are often far more efficient than using a foreach loop.