ref parameters

In this chapter you will learn:

Use ref to pass a value type by reference
Swap two values
Swap two references
Change string using ref keyword

Pass integer by ref

To pass by reference we use the ref parameter modifier. We add the ref modifier in front of the parameter.

The ref parameter modifier causes C# to create a call-by-reference, rather than a call-by-value.

using System;/*j  av a  2 s .c o m*/

class MainClass
{
  static void Main(string[] args)
  {
      int MyInt = 5;

    MyMethodRef(ref MyInt);
   
        Console.WriteLine(MyInt);
  }
  
  static public int MyMethodRef(ref int myInt)
  {
    myInt = myInt + myInt;
    return myInt;
  }
}

The code above generates the following result.

The ref keyword is required in the method declaration as well as the method call.

From the output we can see that the int value in the main method is changed by the method MyMethodRef since the pointer to the int value is passed into the MyMethodRef method.

Swap two values

We can use the ref parameter modifier to exchange value of two parameters.

using System; /*from  j a v  a2 s. c  om*/
 
class Swap { 
  // This method now changes its arguments. 
  public void swap(ref int a, ref int b) { 
    int t; 
  
    t = a; 
    a = b; 
    b = t; 
  } 
} 
 
class MainClass { 
  public static void Main() { 
    Swap ob = new Swap(); 
 
    int x = 10, y = 20; 
 
    Console.WriteLine("x and y before call: " + x + " " + y); 
 
    ob.swap(ref x, ref y);  
 
    Console.WriteLine("x and y after call: " + x + " " + y); 
  } 
}

The code above generates the following result.

Use ref with reference types

We can use ref modifier for reference type value as well.

using System;//  ja v a 2 s . c om
class Rectangle
{
    public int Width = 5;
    public int Height = 5;

}

class Program
{
    static void change(ref Rectangle r)
    {
        r = null;
    }

    static void Main(string[] args)
    {


        Rectangle r = new Rectangle();
        Console.WriteLine(r.Width);
        change(ref r);
        Console.WriteLine(r == null);
    }
}

The output:

If we use the ref modifier in front of a reference type what we get in the method is the address of the reference or the reference of the reference.

Change string using ref keyword

using System;/*from  jav a  2 s  .com*/

class MainClass
{
  public static void UpperCaseThisString(ref string s)
  {
    s = s.ToUpper();
  }
  public static void Main() 
  {
    string s = "str";
    Console.WriteLine("-> Before: {0}", s);
    UpperCaseThisString(ref s);
    Console.WriteLine("-> After: {0}\n", s);
  }
}