List<T>.AreAllSame()

/// <summary>
///   Checks whether all items in the enumerable are same (Uses <see cref="object.Equals(object)" /> to check for equality)
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="enumerable">The enumerable.</param>
/// <returns>
///   Returns true if there is 0 or 1 item in the enumerable or if all items in the enumerable are same (equal to
///   each other) otherwise false.
/// </returns>
public static bool AreAllSame<T>(this IEnumerable<T> enumerable)
{
    if (enumerable == null) throw new ArgumentNullException(nameof(enumerable));

    using (var enumerator = enumerable.GetEnumerator())
    {
        var toCompare = default(T);
        if (enumerator.MoveNext())
        {
            toCompare = enumerator.Current;
        }

        while (enumerator.MoveNext())
        {
            if (toCompare != null && !toCompare.Equals(enumerator.Current))
            {
                return false;
            }
        }
    }

    return true;
}

if (items.AreAllSame()) ...