C# Passing Parameters
Passing Value-Type Parameters
A value-type variable contains its data directly as opposed to a reference-type variable, which contains a reference to its data. Therefore, passing a value-type variable to a method means passing a copy of the variable to the method. Any changes to the parameter that take place inside the method have no affect on the original data stored in the variable. If you want the called method to change the value of the parameter, you have to pass it by reference, using the ref or out keyword.
The following example demonstrates passing value-type parameters by value. The variable n is passed by value to the method SquareIt. Any changes that take place inside the method have no affect on the original value of the variable.
class PassingValByVal
{
static void SquareIt(int x)
// The parameter x is passed by value.
// Changes to x will not affect the original value of x.
{
x *= x;
System.Console.WriteLine("The value inside the method: {0}", x);
}
static void Main()
{
int n = 5;
System.Console.WriteLine("The value before calling the method: {0}", n);
SquareIt(n); // Passing the variable by value.
System.Console.WriteLine("The value after calling the method: {0}", n);
// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
/* Output:
The value before calling the method: 5
The value inside the method: 25
The value after calling the method: 5
*/
The variable n, being a value type, contains its data, the value 5. When SquareIt is invoked, the contents of n are copied into the parameter x, which is squared inside the method. In Main, however, the value of n is the same, before and after calling the SquareIt method. In fact, the change that takes place inside the method only affects the local variable x.
The following example is the same as the previous example, except for passing the parameter using the ref keyword. The value of the parameter is changed after calling the method.
class PassingValByRef
{
static void SquareIt(ref int x)
// The parameter x is passed by reference.
// Changes to x will affect the original value of x.
{
x *= x;
System.Console.WriteLine("The value inside the method: {0}", x);
}
static void Main()
{
int n = 5;
System.Console.WriteLine("The value before calling the method: {0}", n);
SquareIt(ref n); // Passing the variable by reference.
System.Console.WriteLine("The value after calling the method: {0}", n);
// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
/* Output:
The value before calling the method: 5
The value inside the method: 25
The value after calling the method: 25
*/
Passing Reference-Type Parameters
A variable of a reference type does not contain its data directly; it contains a reference to its data. When you pass a reference-type parameter by value, it is possible to change the data pointed to by the reference, such as the value of a class member. However, you cannot change the value of the reference itself; that is, you cannot use the same reference to allocate memory for a new class and have it persist outside the block. To do that, pass the parameter using the ref or out keyword.
The following example demonstrates passing a reference-type parameter, arr, by value, to a method, Change. Because the parameter is a reference to arr, it is possible to change the values of the array elements. However, the attempt to reassign the parameter to a different memory location only works inside the method and does not affect the original variable, arr.
class PassingRefByVal
{
static void Change(int[] pArray)
{
pArray[0] = 888; // This change affects the original element.
pArray = new int[5] {-3, -1, -2, -3, -4}; // This change is local.
System.Console.WriteLine("Inside the method, the first element is: {0}", pArray[0]);
}
static void Main()
{
int[] arr = {1, 4, 5};
System.Console.WriteLine("Inside Main, before calling the method, the first element is: {0}", arr [0]);
Change(arr);
System.Console.WriteLine("Inside Main, after calling the method, the first element is: {0}", arr [0]);
}
}
/* Output:
Inside Main, before calling the method, the first element is: 1
Inside the method, the first element is: -3
Inside Main, after calling the method, the first element is: 888
*/
In the preceding example, the array, arr, which is a reference type, is passed to the method without the ref parameter. In such a case, a copy of the reference, which points to arr, is passed to the method. The output shows that it is possible for the method to change the contents of an array element, in this case from 1 to 888. However, allocating a new portion of memory by using the new operator inside the Change method makes the variable pArray reference a new array. Thus, any changes after that will not affect the original array, arr, which is created inside Main. In fact, two arrays are created in this example, one inside Main and one inside the Change method.
This example is the same as the previous example, except for using the ref keyword in the method header and call. Any changes that take place in the method will affect the original variables in the calling program.
class PassingRefByRef
{
static void Change(ref int[] pArray)
{
// Both of the following changes will affect the original variables:
pArray[0] = 888;
pArray = new int[5] {-3, -1, -2, -3, -4};
System.Console.WriteLine("Inside the method, the first element is: {0}", pArray[0]);
}
static void Main()
{
int[] arr = {1, 4, 5};
System.Console.WriteLine("Inside Main, before calling the method, the first element is: {0}", arr[0]);
Change(ref arr);
System.Console.WriteLine("Inside Main, after calling the method, the first element is: {0}", arr[0]);
}
}
/* Output:
Inside Main, before calling the method, the first element is: 1
Inside the method, the first element is: -3
Inside Main, after calling the method, the first element is: -3
*/
All of the changes that take place inside the method affect the original array in Main. In fact, the original array is reallocated using the new operator. Thus, after calling the Change method, any reference to arr points to the five-element array, which is created in the Change method.
Swapping strings is a good example of passing reference-type parameters by reference. In the example, two strings, str1 and str2, are initialized in Main and passed to the SwapStrings method as parameters modified by the ref keyword. The two strings are swapped inside the method and inside Main as well.
class SwappingStrings
{
static void SwapStrings(ref string s1, ref string s2)
// The string parameter is passed by reference.
// Any changes on parameters will affect the original variables.
{
string temp = s1;
s1 = s2;
s2 = temp;
System.Console.WriteLine("Inside the method: {0} {1}", s1, s2);
}
static void Main()
{
string str1 = "John";
string str2 = "Smith";
System.Console.WriteLine("Inside Main, before swapping: {0} {1}", str1, str2);
SwapStrings(ref str1, ref str2); // Passing strings by reference
System.Console.WriteLine("Inside Main, after swapping: {0} {1}", str1, str2);
}
}
/* Output:
Inside Main, before swapping: John Smith
Inside the method: Smith John
Inside Main, after swapping: Smith John
*/