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Mastering C# for .NET Developers

IEnumerable Interface

Explore how the IEnumerable and IEnumerable<T> interfaces work to support iteration over collections using the foreach loop in C#. Understand the roles of IEnumerator and IEnumerator<T> interfaces, learn to implement custom enumerable collections, and use the yield keyword to simplify iteration logic in your custom types.

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While standard for loops require explicit iteration logic, the foreach loop relies on an underlying structural contract to traverse a collection. To allow a custom type to be iterated using a foreach loop, that type must implement the IEnumerable interface.

Multiple interface implementation
Multiple interface implementation

The IEnumerable<T> interface

Non-generic collections implement the IEnumerable interface, while generic collections use the generic version, IEnumerable<T>. The IEnumerable<T> interface is derived from IEnumerable, and therefore a class implementing the generic version must also implement the non-generic version.

The interfaces are defined as follows:

public interface IEnumerable
{
    IEnumerator GetEnumerator();
}
public interface IEnumerable<out T> : IEnumerable
{
    new IEnumerator<T> GetEnumerator();
}
Definitions of the IEnumerable interfaces

  • Lines 1–4: We define the non-generic interface which requires a method to return an IEnumerator.

  • Lines 6–9: We define the generic interface which inherits from the non-generic version and returns a generic IEnumerator<T>.

When implementing these, we need two GetEnumerator methods, and the one from the non-generic IEnumerable must be implemented explicitly. To understand how the iteration actually occurs, we must examine the object returned by these methods.

The IEnumerator

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