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 (§5.3.3) 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.

A method declared as an iterator (§10.14) cannot have reference parameters.

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.