class TestForeach: CollectionBase
    {
        public void Add(string val)
        {
            List.Add(val);
        }

        public void remove(string val)
        {
            List.Remove(val);
        }
    }
}
//调用TestForeach的测试代码如下：
public void test()
{
            TestForeach cols = new TestForeach();
            cols.Add("Name");
            cols.Add("Sex");

            foreach (string col in cols)
            {
                MessageBox.Show("hello!" + col);
            }
}

        public IEnumerable Power(int number, int exponent) 
        { 
            int counter = 0;
            int result = 1; 
            while (counter++ < exponent) 
            { 
                result = result * number;
                yield return result;
            } 
        }

         private void testYield()
         {   
             // Display powers of 2 up to the exponent 8:        
             foreach (int i in Power(2, 8))
             {            
                 //Console.Write("{0} ", i);
                 MessageBox.Show("hello!" + i);
             }    
         }