C#中foreach循环对比for循环的优势和劣势

int[,] nVisited ={
  {1,2,3},
  {4,5,6},
  {7,8,9}
};
// Use "for" to loop two-dimension array(使用for循环二维数组)
Console.WriteLine("User 'for' to loop two-dimension array");
for (int i = 0; i < nVisited.GetLength(0); i++)
 for (int j = 0; j < nVisited.GetLength(1); j++)
   Console.Write(nVisited[i, j]);
   Console.WriteLine();

//Use "foreach" to loop two-dimension array(使用foreach循环二维数组)
Console.WriteLine("User 'foreach' to loop two-dimension array");
foreach (var item in nVisited)
Console.Write(item.ToString());

int[][] nVisited = new int[3][];
nVisited[0] = new int[3] { 1, 2, 3 };
nVisited[1] = new int[3] { 4, 5, 6 };
nVisited[2] = new int[6] { 1, 2, 3, 4, 5, 6 };

//Use "foreach" to loop two-dimension array(使用foreach循环二维数组)
Console.WriteLine("User 'foreach' to loop two-dimension array");
foreach (var item in nVisited)
  foreach (var val in item)
   Console.WriteLine(val.ToString());

int[] val = { 1, 2, 3 };
ArrayList list = new ArrayList();
list.AddRange(val);
foreach (int item in list)//在循环语句中指定当前正在循环的元素的类型,不需要进行拆箱转换
{
Console.WriteLine((2*item));
}
Console.WriteLine();
for (int i = 0; i < list.Count; i++)
{
int item = (int)list[i];//for循环需要进行拆箱
Console.WriteLine(2 * item);
}

using (IEnumerator<T> enumerator = collection.GetEnumerator())
{
  while (enumerator.MoveNext())
  {
   this.Add(enumerator.Current);
  }
}