C# 0 The Complete Reference

Use ref and out Parameters

As just explained, value types, such as 

int

 or 

char

, are passed by value to a method. This 

means that changes to the parameter that receives a value type will not affect the actual 

argument used in the call. You can, however, alter this behavior. Through the use of the 

ref

and

out

 keywords, it is possible to pass any of the value types by reference. Doing so allows 

a method to alter the argument used in the call.

Before going into the mechanics of using 

ref

 and 

out

, it is useful to understand why you 

might want to pass a value type by reference. In general, there are two reasons: to allow a 

method to alter the contents of its arguments or to allow a method to return more than one 

value. Let’s look at each reason in detail.

Often you will want a method to be able to operate on the actual arguments that are 

passed to it. The quintessential example of this is a 

Swap( )

 method that exchanges the values 

of its two arguments. Since value types are passed by value, it is not possible to write a 

method that swaps the value of two 

int

s, for example, using C#’s default call-by-value 

parameter passing mechanism. The 

ref

 modifier solves this problem.

As you know, a 

return

 statement enables a method to return a value to its caller. However, a 

method can return 

only one

 value each time it is called. What if you need to return two or more 

pieces of information? For example, what if you want to create a method that decomposes a 

floating-point number into its integer and fractional parts? To do this requires that two pieces 

of information be returned: the integer portion and the fractional component. This method 

cannot be written using only a single return value. The 

out

 modifier solves this problem.

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