Type perfect Object potx

Boxing As you have just seen, variables of type object can refer to any object of any reference type. That is, they can refer to any instance of a class. However, variables of type object can also refer to a value type. For example, the following two statements initialize the variable i (of type int, a value type) to 42 and then initialize the variable o (of type object, a reference type) to i: int i = 42; object o = i; The effect of the second statement is subtle. Remember that i is a value type and it exists in the stack. If the reference inside o referred directly to i, the reference would be referring to the stack. However, all references refer to objects on the heap. Instead, a piece of memory is allocated from the heap, an exact copy of the value inside i is stored in this piece of memory, and the reference inside o is pointed to the copy. (Creating references to items on the stack could seriously compromise the robustness of the runtime, and create a potential security flaw, so it is not allowed.) This automatic copying of an item from the stack to the heap is called boxing. The following graphic shows the result: IMPORTANT If you modify the original value of a variable, you are not modifying the value that now exists on the heap, because it's just a copy. Calling Methods Methods exist to be called! You call a method by name to ask it to perform its task. If the method requires information (as specified by its parameters), you must supply the information requested. If the method returns information (as specified by its return type), you should arrange to capture this information somehow. Specifying the Method Call Syntax The syntax of a C# method call is as follows: methodName ( argumentList ) • The methodName must exactly match the name of the method you're calling. Remember, C# is a case-sensitive language. • The argumentList supplies the optional information that the method accepts. You must supply an argument for each parameter, and the value of each argument must be compatible with the type of its corresponding parameter. If the method you're calling has two or more parameters, you must separate the arguments with commas. IMPORTANT You must include the parentheses in every method call, even when calling a method that has no arguments. Here is the addValues method again: int addValues(int leftHandSide, int rightHandSide) { // } The addValues method has two int parameters, so you must call it with two comma- separated int arguments: addValues(39, 3); // okay You can also replace the literal values 39 and 3 with the names of int variables. The values in those variables are then passed to the method as its arguments, like this: int arg1 = 99; int arg2 = 1; addValues(arg1, arg2); If you try to call addValues in some other way, you will probably not succeed, for the reasons described in the following examples: addValues; // compile time error, no parentheses addValues(); // compile time error, not enough arguments addValues(39); // compile time error, not enough arguments addValues("39", "3"); // compile time error, wrong types The addValues method returns an int value. This int value can be used wherever an int value can be used. Consider these examples: result = addValues(39, 3); // on right hand side of an assignment showResult(addValues(39, 3)); // as argument to another method call The following exercise continues using the MathsOperators application. This time you will examine some method calls. Examine method calls 1. Return to the Methods project. (This project is already open in Visual Studio 2005 if you're continuing from the previous exercise. If you are not, open it from the \Microsoft Press\Visual CSharp Step by Step\Chapter 3\Methods folder in your My Documents folder. 2. Display the code for Form1.cs in the Code and Text Editor window. 3. Locate the calculate_Click method, and look at the first two statements of this method after the try statement and opening curly brace. (We will cover the purpose of try statements in Chapter 6, “Managing Errors and Exceptions.”) The statements are as follows: int leftHandSide = System.Int32.Parse(leftHandSideOperand.Text); int rightHandSide = System.Int32.Parse(rightHandSideOperand.Text); These two statements declare two int variables called leftHandSide and rightHandSide. However, the interesting parts are the way in which the variables are initialized. In both cases, the Parse method of the System.Int32 class is called (System is a namespace, Int32 is the name of the class in this namespace). This method takes a single string parameter and converts it to an int value. These two lines of code take whatever the user has typed into the leftHandSideOperand and rightHandSideOperand TextBox controls on the form and converts them into int values. 4. Look at the fourth statement in the calculate_Click method (after the if statement and another opening curly brace): calculatedValue = addValues(leftHandSide, rightHandSide)); This statement calls the addValues method, passing the values of the leftHandSide and rightHandSide variables as its arguments. The value returned by the addValues method is stored in the calculatedValue variable. 5. Look at the next statement: showResult(calculatedValue); This statement calls the showResult method, passing the value in the calculatedValue variable as its argument. The showResult method does not return a value. 6. In the Code and Text Editor window, find the showResult method you looked at earlier. The only statement of this method is this: result.Text = answer.ToString(); Notice that the ToString method call uses parentheses even though there are no arguments. TIP You can call methods belonging to other objects by prefixing the method with the name of the object. In the previous example, the expression answer.ToString() calls the method named ToString belonging to the object called answer. . variables of type object can refer to any object of any reference type. That is, they can refer to any instance of a class. However, variables of type object can also refer to a value type. For. statements initialize the variable i (of type int, a value type) to 42 and then initialize the variable o (of type object, a reference type) to i: int i = 42; object o = i; The effect of the second. belonging to other objects by prefixing the method with the name of the object. In the previous example, the expression answer.ToString() calls the method named ToString belonging to the object called