which interface must be implemented to the class to use foreach loop?
IEnumerable and IEnumerator. When using C# with the .NET framework version 1.1, you can create classes that behave like collections or arrays. If you wish to use the foreach loop structure to iterate through the values in your class, you must implement the IEnumerable interface. This interface contains a single method, named GetEnumerator, which returns an enumerator object.