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This chapter is from the book

This chapter is from the book

7.18 Passing Arguments By Value and By Reference

Two ways to pass arguments to methods in many programming languages are pass-by-value and pass-by-reference. When an argument is passed by value (the default in C#), a copy of its value is made and passed to the called method. Changes to the copy do not affect the original variable’s value in the caller. This prevents the accidental side effects that so greatly hinder the development of correct and reliable software systems. Each argument that’s been passed in the programs so far has been passed by value. When an argument is passed by reference, the caller gives the method the ability to access and modify the caller’s original variable—no copy is passed.

To pass an object by reference into a method, simply provide as an argument in the method call the variable that refers to the object. Then, in the method body, reference the object using the corresponding parameter name. The parameter refers to the original object in memory, so the called method can access the original object directly.

In the previous section, we began discussing the differences between value types and reference types. A major difference is that:

  • value-type variables store values, so specifying a value-type variable in a method call passes a copy of that variable’s value to the method, whereas

  • reference-type variables store references to objects, so specifying a reference-type variable as an argument passes the method a copy of the reference that refers to the object.

Even though the reference itself is passed by value, the method can still use the reference it receives to interact with—and possibly modify—the original object. Similarly, when returning information from a method via a return statement, the method returns a copy of the value stored in a value-type variable or a copy of the reference stored in a reference-type variable. When a reference is returned, the calling method can use that reference to interact with the referenced object.

7.18.1 ref and out Parameters

What if you would like to pass a variable by reference so the called method can modify the variable’s value in the caller? To do this, C# provides keywords ref and out.

ref Parameters

Applying the ref keyword to a parameter declaration allows you to pass a variable to a method by reference—the method will be able to modify the original variable in the caller. Keyword ref is used for variables that already have been initialized in the calling method.

out Parameters

Preceding a parameter with keyword out creates an output parameter. This indicates to the compiler that the argument will be passed into the called method by reference and that the called method will assign a value to the original variable in the caller. This also prevents the compiler from generating an error message for an uninitialized variable that’s passed as an argument to a method.

Passing Reference-Type Variables by Reference

You also can pass a reference-type variable by reference, which allows you to modify it so that it refers to a new object. Passing a reference by reference is a tricky but powerful technique that we discuss in Section 8.13.

7.18.2 Demonstrating ref, out and Value Parameters

The app in Fig. 7.19 uses the ref and out keywords to manipulate integer values. The class contains three methods that calculate the square of an integer.

 1   // Fig. 7.19: ReferenceAndOutputParameters.cs
 2   // Reference, output and value parameters.
 3   using System;
 4
 5   class ReferenceAndOutputParameters
 6   {
 7      // call methods with reference, output and value parameters
 8      static void Main()
 9      {
10         int y = 5; // initialize y to 5
11         int z; // declares z, but does not initialize it
12
13         // display original values of y and z
14         Console.WriteLine($"Original value of y: {y}");
15         Console.WriteLine("Original value of z: uninitialized\n");
16
17         // pass y and z by reference
18         SquareRef(ref y); // must use keyword ref
19         SquareOut(out z); // must use keyword out
20
21         // display values of y and z after they're modified by
22         // methods SquareRef and SquareOut, respectively
23         Console.WriteLine($"Value of y after SquareRef: {y}");
24         Console.WriteLine($"Value of z after SquareOut: {z}\n");
25
26         // pass y and z by value
27         Square(y);
28         Square(z);
29
30         // display values of y and z after they're passed to method Square
31         // to demonstrate that arguments passed by value are not modified
32         Console.WriteLine($"Value of y after Square: {y}");
33         Console.WriteLine($"Value of z after Square: {z}");
34      }
35
36      // uses reference parameter x to modify caller's variable
37      static void SquareRef(ref int x)
38      {
39         x = x * x; // squares value of caller's variable
40      }
41
42      // uses output parameter x to assign a value
43      // to an uninitialized variable
44      static void SquareOut(out int x)
45      {
46         x = 6; // assigns a value to caller's variable
47         x = x * x; // squares value of caller's variable
48      }
49
50      // parameter x receives a copy of the value passed as an argument,
51      // so this method cannot modify the caller's variable
52      static void Square(int x)
53      {
54         x = x * x;
55      }
56   }
Original value of y: 5
Original value of z: uninitialized

Value of y after SquareRef: 25
Value of z after SquareOut: 36

Value of y after Square: 25
Value of z after Square: 36

Fig. 7.19 | Reference, output and value parameters.

Method SquareRef (lines 37–40) multiplies its parameter x by itself and assigns the new value to x. SquareRef’s parameter is declared as ref int, which indicates that the argument passed to this method must be an integer that’s passed by reference. Because the argument is passed by reference, the assignment at line 39 modifies the original argument’s value in the caller.

Method SquareOut (lines 44–48) assigns its parameter the value 6 (line 46), then squares that value. SquareOut’s parameter is declared as out int, which indicates that the argument passed to this method must be an integer that’s passed by reference and that the argument does not need to be initialized in advance.

Method Square (lines 52–55) multiplies its parameter x by itself and assigns the new value to x. When this method is called, a copy of the argument is passed to the parameter x. Thus, even though parameter x is modified in the method, the original value in the caller is not modified.

Method Main (lines 8–34) invokes methods SquareRef, SquareOut and Square. We  begin by initializing variable y to 5 and declaring, but not initializing, variable z. Lines 18–19 call methods SquareRef and SquareOut. Notice that when you pass a variable to a method with a reference parameter, you must precede the argument with the same keyword (ref or out) that was used to declare the reference parameter. Lines 23–24 display the values of y and z after the calls to SquareRef and SquareOut. Notice that y has been changed to 25 and z has been set to 36.

Lines 27–28 call method Square with y and z as arguments. In this case, both variables are passed by value—only copies of their values are passed to Square. As a result, the values of y and z remain 25 and 36, respectively. Lines 32–33 output the values of y and z to show that they were not modified.

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