IEnumerable and IEnumerator in C#: How Do They Enable foreach Loops and Custom Iterators?

In-depth understanding of IEnumerable and IEnumerator: the core mechanism of .NET iterators

In .NET programming, implementing the IEnumerable interface is the key to enabling foreach loops. Although the foreach syntax is concise and easy to use, its underlying dependence is on the IEnumerable and IEnumerator interfaces.

Application scenarios of IEnumerable and IEnumerator

IEnumerable is not used to "replace" foreach; on the contrary, implementing IEnumerable is a prerequisite for using foreach. When you write a foreach loop, the compiler actually converts it into a series of calls to the IEnumerator's MoveNext and Current methods.

The difference between IEnumerable and IEnumerator

• IEnumerable: defines an interface with the GetEnumerator method, which returns an IEnumerator.
• IEnumerator: defines the interface of MoveNext and Current methods. MoveNext moves the iterator to the next item in the sequence, while Current returns the current item.

Why do we need IEnumerable and IEnumerator?

• Custom Iterator: By implementing IEnumerable on your custom classes, you can use a foreach loop to iterate over those classes.
• Understanding the iterator mechanism: IEnumerator provides a low-level mechanism for traversing a collection, allowing you to control the traversal process.
• Generic Collections: The C# compiler uses IEnumerable and IEnumerator to create generic collection classes, such as List and Dictionary.
• Asynchronous Programming: IEnumerator can be used in conjunction with the asynchronous programming model to traverse asynchronous sequences.

implements IEnumerable

To make a class iterable, you need to implement the IEnumerable interface, which requires implementing the GetEnumerator method that returns an IEnumerator.

Example

Consider the following custom object class that implements the IEnumerable interface:

public class CustomCollection : IEnumerable<int>
{
    private int[] data;

    public IEnumerator<int> GetEnumerator()
    {
        return new CustomEnumerator(data);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }

    private class CustomEnumerator : IEnumerator<int>
    {
        // MoveNext 和 Current 方法用于遍历 data
    }
}

By implementing IEnumerable and providing a custom enumerator, you can now use a foreach loop to iterate over instances of a CustomCollection.

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