I wrote the algorithm after mock interview discussion with the interviewer. It is the first time I learned the idea and it took me too long in the mock interview to understand the idea.

Leetcode54_Spiralmatrix_AfterMockInterview.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MatrixSpiralPrint
{
    class Program
    {
        /// <summary>
        /// Leetcode Spiral Matrix
        /// https://leetcode.com/problems/spiral-matrix/description/
        /// </summary>
        /// <param name="args"></param>
        static void Main(string[] args)
        {
            var spiral = MatrixSpiralPrint(new int[,]{{1, 2, 3}, {8, 9, 4}, {7, 6, 5}});
            Debug.Assert(String.Join(",", spiral).CompareTo("123456789") == 0); 
        }

        /// <summary>
        /// code written on January 24, 2018
        /// Practice after the mock interview on January 23, 2018
        /// The interviewer asked me to work on the simple code, navigate the direction 
        /// automatically by checking boudary and checking the status of element visited
        /// status. 
        /// Only one loop, only 20 minutes code; so the presentation will be much better
        /// and more interesting. 
        /// </summary>
        /// <param name="array"></param>
        /// <returns></returns>
        public static int[] MatrixSpiralPrint(int[,] array)
        {
            if (array == null || array.GetLength(0) == 0 || array.GetLength(1) == 0)
            {
                return new int[0];
            }

            int rows    = array.GetLength(0);
            int columns = array.GetLength(1); 

            var visited = new int[rows, columns];
            int index = 0;
            int totalNumbers = rows * columns;

            var fourDirections = new List<int[]>();

            fourDirections.Add(new int[] {  0,  1 });    // left to right - top row
            fourDirections.Add(new int[] {  1,  0 });    // top to down   - last column
            fourDirections.Add(new int[] {  0, -1 });    // right to left - bottom row
            fourDirections.Add(new int[] { -1,  0 });    // bottom up     - first row

            int direction = 0;
            int row = 0;
            int col = 0; 

            var spiral = new int[totalNumbers]; 

            while (index < totalNumbers)
            {
                var current = array[row, col];
                
                spiral[index++] = current;

                visited[row, col] = 1; // mark visit

                var nextRow = row + fourDirections[direction][0];
                var nextCol = col + fourDirections[direction][1];

                var isOutArrayBoundary = nextRow < 0 || nextRow >= rows || nextCol < 0 || nextCol >= columns;

                if (isOutArrayBoundary || visited[nextRow, nextCol] == 1) // change the direction
                {
                    direction = (direction + 1) % 4; // map to 0 to 3                    
                }

                row += fourDirections[direction][0];
                col += fourDirections[direction][1];
            }

            return spiral;
        }
    }
}