C# 5.0 Programmer's Reference

84 ❘ CHAPTER 4 Data Types, Variables, and Constants
If you run the code now, the following text appears in the Console window.
DoubleIt: 20
DoDouble: 20
This time the DoubleIt method doubled its parameter, and the change was reflected in the value
argument in the calling code.
For Output
The final way you can pass a value into a method uses the out keyword. This keyword means the
parameter is intended to be an output parameter. The argument is passed into the method by reference
so the method can set its value. The method does not assume the value has been initialized before it is
passed into the method, and the method assigns a value to the parameter before it returns.
When Should You Pass for Output
You should pass parameters for output whenever you want a method to initialize a
parameter but the method doesn’t need to use the parameter’s initial value.
For example, suppose a method fetches data about a particular model of car and
uses output parameters to return information about the model such list price, miles
per gallon, horsepower, and number of cup holders. In that case, the method initializes
those values before it returns but doesn’t use any values that those parameters
might have on input, so the parameters should be declared for output.
If you have the choice between declaring a parameter by reference or for output,
declare it for output because that is more restrictive and may help you catch bugs.
When you declare a parameter for output, Visual Studio does not require the calling
code to initialize the parameter passed into the method. It also flags any code that
uses the parameter’s input value as an error because marking the parameter for output
indicates that its input value is not needed.
As is the case for the ref keyword, if you add the out keyword to a parameter declaration, you must
also include it with the argument. The following code shows the previous example modified to use
the out keyword.
private void DoubleTest()
{
int value;
DoubleIt(out value);
Console.WriteLine("DoubleTest: " + value.ToString());
}
private void DoubleIt(out int number)
{
number = 50;
Console.WriteLine("DoubleIt: " + number.ToString());
}
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