What Makes a C# Class Iterable Using a Foreach Loop?

In-depth understanding of the foreach loop in C#

The foreach loop in C# is a powerful language construct that allows developers to iterate over the elements of a collection easily and efficiently. But what exactly does a class need to meet in order to use the foreach loop?

Iterable class

Any class that implements the IEnumerable interface can be iterated using a foreach loop. Technically, the class only needs to provide a MoveNext() method that returns an object with a Current method and a GetEnumerator() property, but implementing IEnumerable is the most common way to provide this functionality.

Common collection types

Standard collection types, such as Collection<t></t>, List<t></t>, and arrays (T[]), all implement the ICollection or IEnumerable interface, allowing their use in foreach loops. This is because these types represent collections of data that can be enumerated.

How the foreach loop works

A foreach (int i in obj) loop similar to foreach roughly translates to the following code:

var tmp = obj.GetEnumerator();
int i; // C# 4.0 之前
while (tmp.MoveNext())
{
    int i; // C# 5.0 及以后
    i = tmp.Current;
    // 你的代码
}

tmp The variable stores the enumerator, which is responsible for traversing the collection. On each iteration, the MoveNext() method advances the enumerator to the next element, while the Current attribute returns the current element (which is assigned to the loop variable, such as i in our example).

Variations and Releases

Depending on implementation, enumerators may support the IDisposable interface. If so, C# will automatically release the enumerator using the using pattern when the loop exits. It is important to note that in C# 5.0, the declaration of the loop variable (int i) was moved inside the loop, which is crucial if you plan to use the variable in an anonymous method or lambda expression.

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