ECMA-334 C# Language Specification

17.5.1.2: Reference parameters

A parameter declared with a ref modifier is a reference parameter. Unlike a value parameter, a reference parameter does not create a new storage location. Instead, a reference parameter represents the same storage location as the variable given as the argument in the method invocation.

When a formal parameter is a reference parameter, the corresponding argument in a method invocation must consist of the keyword ref followed by a variable-reference (12.3.3) of the same type as the formal parameter. A variable must be definitely assigned before it can be passed as a reference parameter.

Within a method, a reference parameter is always considered definitely assigned.

[Example: The example 
using System;  
class Test  
{  
   static void Swap(ref int x, ref int y) {  
      int temp = x;  
      x = y;  
      y = temp;  
   }  
   static void Main() {  
      int i = 1, j = 2;  
      Swap(ref i, ref j);  
      Console.WriteLine("i = {0}, j = {1}", i, j);  
   }  
}
produces the output 
i = 2, j = 1

For the invocation of Swap in Main, x represents i and y represents j. Thus, the invocation has the effect of swapping the values of i and j. end example]

In a method that takes reference parameters, it is possible for multiple names to represent the same storage location. [Example: In the example 
class A  
{  
   string s;  
   void F(ref string a, ref string b) {  
      s = "One";  
      a = "Two";  
      b = "Three";  
   }  
   void G() {  
      F(ref s, ref s);  
   }  
}
the invocation of F in G passes a reference to s for both a and b. Thus, for that invocation, the names s, a, and b all refer to the same storage location, and the three assignments all modify the instance field s. end example]