Constructors

1. Constructors

2. Constructors • You got plenty of experience using constructors in the Marching Band program. • A constructor is the subroutine which creates objects from a class. • Every class has a default constructor which takes no parameters– New(). • This default constructor is there even if you don’t see it.

3. ThreeDPoint • In the class definition, there is no Sub New. • However, I can still call the default constructor like this:

4. Creating a Constructor • However, we can create our own constructor. • There are two main reasons for creating your own constructor: • To initialize the value of variables using parameters; and • To create any objects that the current object will need; that is, calling constructors of other classes. Here’s the Rank class’s constructor from Marching Band: • Notice that it • Initializes the mStepSize variable; • Creates the 10 members of the rank by calling BandMember’s constructor.

5. Creating a Constructor • Here’s a simple constructor for the ThreeDPoint class. It simply initializes the variables.

6. Constructors in Action • To understand how to declare variables and create objects using constructors, • View the ConstructorsInAction video.

7. Overloads • VB allows functions and subs (including constructors) to be overloaded. • This means that you can have two or more procedures with the same name. • They just have to differ in the number and/or type of parameters. • Overloads are used widely in the built-in VB.NET classes (you’ll see many in the graphics routines), but you can create your own overloads as well.

8. Overload Example

9. ToString • Older versions of VB, including Visual Basic for Applications in Excel, do not have a “ToString” function. • VBA is a loosely-typed language that does all sorts of datatype conversions in the background for you (sometimes well, sometimes not). • VB.NET, the descendant of VBA (VB 6), is a strongly typed language, which means that functions, subs and properties can require a particular parameter type. • If a String parameter type is required you can put ToString on the end of a variable or expression to convert it to a string.

10. ToString • For example: Notice that when I try to put a numeric value into a Text property, I get an error: • ToString is the cure: • (Note: I added the extra blank lines so the error message wouldn’t cover up the line above. This doesn’t mean that I now approve of lots of blank lines!)

11. ToString • The standard datatypes (even String) have built-in ToString methods. • ToString can even be applies to expressions: • However, the classes that you create will have a default ToString method that’s pretty useless: It just displays the name of the class. • And this can be annoying when you start adding objects of your own classes to ListBoxes and ComboBoxes!

12. Using ToString to Format Numbers • ToString can be used to format numbers, like this: