C# 4 and .NET 4

Classes ❘ 71
out Parameters
In C-style languages, it is common for functions to be able to output more than one value from a single
routine. This is accomplished using output parameters, by assigning the output values to variables that
have been passed to the method by reference. Often, the starting values of the variables that are passed by
reference are unimportant. Those values will be overwritten by the function, which may never even look at
any previous value.
It would be convenient if you could use the same convention in C#. However, C# requires that variables
be initialized with a starting value before they are referenced. Although you could initialize your input
variables with meaningless values before passing them into a function that will fill them with real,
meaningful ones, this practice seems at best needless and at worst confusing. However, there is a way to
short-circuit the C# compiler’s insistence on initial values for input arguments.
You do this with the out keyword. When a method’s input argument is prefixed with out, that method can
be passed a variable that has not been initialized. The variable is passed by reference, so any changes that
the method makes to the variable will persist when control returns from the called method. Again, you also
need to use the out keyword when you call the method, as well as when you define it:
static void SomeFunction(out int i)
{
i = 100;
}
public static int Main()
{
int i; // note how i is declared but not initialized.
SomeFunction(out i);
Console.WriteLine(i);
return 0;
}
Named Arguments
Typically, parameters need to be passed into a method in the same order that they are defined. Named
arguments allow you to pass in parameters in any order. So for the following method:
string FullName(string firstName, string lastName)
{
return firstName + " " + lastName;
}
The following method calls will return the same full name:
FullName("John", "Doe");
FullName(lastName: "Doe", firstName: "John");
If the method has several parameters, you can mix positional and named arguments in the same call.
Optional Arguments
Parameters can also be optional. You must supply a default value for parameters that are optional. The
optional parameter(s) must be the last ones defined as well. So the following method declaration would
be incorrect:
void TestMethod(int optionalNumber = 10, int notOptionalNumber)
{
System.Console.Write(optionalNumber + notOptionalNumber);
}
For this method to work, the optionalNumber parameter would have to be defined last.
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