Parameters

Parameters are used to pass values or variable references to methods. The parameters of a method get their actual values from the arguments that are specified when the method is invoked. There are four kinds of parameters: value parameters, reference parameters, output parameters, and parameter arrays.

A value parameter is used for input parameter passing. A value parameter corresponds to a local variable that gets its initial value from the argument that was passed for the parameter. Modifications to a value parameter do not affect the argument that was passed for the parameter.

Value parameters can be optional, by specifying a default value so that corresponding arguments can be omitted.

A reference parameter is used for both input and output parameter passing. The argument passed for a reference parameter must be a variable, and during execution of the method, the reference parameter represents the same storage location as the argument variable. A reference parameter is declared with the ref modifier. The following example shows the use of ref parameters.

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("{0} {1}", i, j); // Outputs "2 1"
}
}

An output parameter is used for output parameter passing. An output parameter is similar to a reference parameter except that the initial value of the caller-provided argument is unimportant. An output parameter is declared with the out modifier. The following example shows the use of out parameters.

using System;

class Test
{
static void Divide(int x, int y, out int result, out int remainder) {
result = x / y;
remainder = x % y;
}

static void Main() {
int res, rem;
Divide(10, 3, out res, out rem);
Console.WriteLine("{0} {1}", res, rem); // Outputs "3 1"
}
}

A parameter array permits a variable number of arguments to be passed to a method. A parameter array is declared with the params modifier. Only the last parameter of a method can be a parameter array, and the type of a parameter array must be a single-dimensional array type. The Write and WriteLine methods of the System.Console class are good examples of parameter array usage. They are declared as follows.

public class Console
{
public static void Write(string fmt, params object[] args) {...}

public static void WriteLine(string fmt, params object[] args) {...}

...
}

Within a method that uses a parameter array, the parameter array behaves exactly like a regular parameter of an array type. However, in an invocation of a method with a parameter array, it is possible to pass either a single argument of the parameter array type or any number of arguments of the element type of the parameter array. In the latter case, an array instance is automatically created and initialized with the given arguments. This example

Console.WriteLine("x={0} y={1} z={2}", x, y, z);

is equivalent to writing the following.

string s = "x={0} y={1} z={2}";
object[] args = new object[3];
args[0] = x;
args[1] = y;
args[2] = z;
Console.WriteLine(s, args);