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 (Section 5.4) 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.

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.

In a method that takes reference parameters it is possible for multiple names to represent the same storage location. 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.