EnumerableExtensions.cs

namespace ApiVersioning.Examples;

using System.Collections;

/// <summary>
/// Provides extension methods for the <see cref="IEnumerable"/> interface.
/// </summary>
public static class EnumerableExtensions
{
    /// <summary>
    /// Returns the first element from the specified sequence.
    /// </summary>
    /// <param name="enumerable">The <see cref="IEnumerable">sequence</see> to take an element from.</param>
    /// <returns>The first element in the sequence or <c>null</c>.</returns>
    public static object FirstOrDefault( this IEnumerable enumerable )
    {
        var iterator = enumerable.GetEnumerator();

        try
        {
            if ( iterator.MoveNext() )
            {
                return iterator.Current;
            }
        }
        finally
        {
            if ( iterator is IDisposable disposable )
            {
                disposable.Dispose();
            }
        }

        return default;
    }

    /// <summary>
    /// Returns a single element from the specified sequence.
    /// </summary>
    /// <param name="enumerable">The <see cref="IEnumerable">sequence</see> to take an element from.</param>
    /// <returns>The single element in the sequence or <c>null</c>.</returns>
    public static object SingleOrDefault( this IEnumerable enumerable )
    {
        var iterator = enumerable.GetEnumerator();
        var result = default( object );

        try
        {
            if ( iterator.MoveNext() )
            {
                result = iterator.Current;

                if ( iterator.MoveNext() )
                {
                    throw new InvalidOperationException( "The sequence contains more than one element." );
                }
            }
        }
        finally
        {
            if ( iterator is IDisposable disposable )
            {
                disposable.Dispose();
            }
        }

        return result;
    }
}