What object types can be used in a foreach loop in C#?

In a foreach loop in C#, you can use any object types that implement one of the following interfaces:

• System.Collections.IEnumerable - For non-generic collections. Provides a non-generic enumerator IEnumerator.
• System.Collections.Generic.IEnumerable<T> - For generic collections. Provides a generic enumerator IEnumerator<T>.

A class or struct can also be used in foreach if it has a public method GetEnumerator() that returns an instance of a class/struct implementing the following signature (this is called pattern-based foreach):

• A public property Current returning any type.
• A public method MoveNext() returning bool.
• Optionally, a public method Dispose() (for proper resource release when using using).

Examples of built-in types that can be used:

• Arrays (int[], string[], etc.)
• Collections from the System.Collections namespace: ArrayList, Hashtable, Queue, Stack.
• Collections from the System.Collections.Generic namespace: List<T>, Dictionary<TKey, TValue>, HashSet<T>, Queue<T>, Stack<T>.
• Strings (string) - they are sequences of char characters.

An example of a custom type supporting foreach based on the pattern:

// Simple custom type supporting foreach
public class MyEnumerable
{
    private int[] data = { 1, 2, 3, 4, 5 };

    // GetEnumerator method supporting foreach pattern
    public MyEnumerator GetEnumerator()
    {
        return new MyEnumerator(data);
    }

    // Internal enumerator class
    public class MyEnumerator
    {
        private int[] data;
        private int currentIndex = -1;

        public MyEnumerator(int[] data)
        {
            this.data = data;
        }

        // Current property
        public int Current => data[currentIndex];

        // MoveNext method
        public bool MoveNext()
        {
            currentIndex++;
            return currentIndex < data.Length;
        }

        // Dispose method (optional)
        public void Dispose()
        {
            // Resource cleanup if necessary
        }
    }
}

// Usage in foreach
// var collection = new MyEnumerable();
// foreach (var item in collection)
// {
//     // Console.WriteLine(item);
// }