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贴子主题： C#快餐－5

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发信人: walts (休息一会), 信区: DotNET
标  题: C#快餐－5
发信站: BBS 水木清华站 (Thu Jul 26 01:25:22 2001)

Lesson 5. Passing by value and by reference

    Here is a simple program that attempts to modify an integer.

using System;
//does not change value of k
class Demo
{
   public class Test
   {
       public void change (int k)
       {
       k=k+2;
       }
   }
   public static void Main()
   {
      int i=3;
      Test hi=new Test();
      Console.WriteLine("Before {0} ", i);
      hi.change(i);
      Console.WriteLine("After {0} ", i);
   }
}

     If you compile and execute this program you will find that
the value of the integer have not been changed by method change.
The reason is quite simple: method change takes a value of i,
makes a copy of it, adds 2 to the copy and returns control to
the main. Value of i does not change.When  change returns,
its local copy of variable i disappears ( goes out of scope).
When  Console.WriteLine("After {0} ",i); is called, it uses i
which was never changed. A simple fix is to modify method change
so that it returns a value:

using System;
// Circumvent the problem of changing value of i.
class Demo
{
    public class Test{
        public int change (int k)
        {
        return k+2;
        }
    }
    public static void Main()
    {
    int i=3;
    Test hi=new Test();
    Console.WriteLine("Before {0} ",i);
    Console.WriteLine("After {0} ", hi.change(i));
    }
}

   The code above works fines but it does not change
the value of i. To change i  we need to pass it by
  reference. When passing by reference compiler
  changes i  instead of changing a local copy.
  To do this we need to add keyword ref  indicating
  that change takes a reference as an argument.

using System;
class Demo
{
   public class Test{
      public void change(ref int k)
      {
         k=k+2;
      }
   }
   public static void Main()
   {
      int i=3;
      Test hi=new Test();
      Console.WriteLine("Before {0} ", i);
      hi.change(ref  i);
      Console.WriteLine("After {0} ", i);
   }
}

The output of this program is:
Before 3
After 5

--

A great poem is a fountain forever overflowing with the
　waters of wisdom and delight.
　　　　　　　　　　　
                                          —— Shelley

※ 来源:·BBS 水木清华站 smth.org·[FROM: 166.111.142.118]
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