.Net Class Diagrams

.Net Class Diagrams

A class diagram is an effective way to map out and clarify the design of a .Net object.  Here’s an example:

A Class Diagram Example

	ExcelWorker				Class Name
	int customerId				Class Field Names with Data Types
	string itemName
	String	ConnectToExcel			Class Methods and Return Values
	DataTable	ReadExcel
	Boolean	UpdateExcel

How to Build from the Class Diagram

1. Add a Class to the Project

In the solution explorer, right click on the project, select add class and provide a descriptive name.  In my example, the class is named ExcelWorker since it will be working with data in a spreadsheet.    

2. Add the Class Fields

Although it is not always apparent up front how many variables and what type will be needed, the class diagram indicates that we at least need an int and a string variable named customerId and customerName respectively.  Class variables are known as “fields.” 

These are declared at the top of the class within the first class brace:

class ExcelWorker

    {

        public int CustomerId;

        public string CustomerName;

    }

These can be initialized in one of two ways: either after the object has been initialized …

ExcelWorker builder = new ExcelWorker();

        

builder.CustomerId = 123;

builder.CustomerName = "Bob";

or they can be initialized when the object is initialized:

ExcelWorker builder = new ExcelWorker() {CustomerId = 123, CustomerName = "Bob"};

The Importance of Class Fields

The fields will keep track of the data for each instance of the class.  For example, if a second ExcelWorker object named ratchet is declared and its fields initialized, the builder and ratchet class objects will have separate field values.

ExcelWorker ratchet = new ExcelWorker() {CustomerId = 456, CustomerName = "Page"};

The CustomerId and CustomerName values for builder will be 123 and “Bob” respectively and 456 and “Page” for ratchet.

3. Add the Class Methods and Return Types

The class diagram indicates that three methods are needed in the class:

Return Type	Method Name
String	ConnectToExcel
DataReader	ReadExcel
Boolean	UpdateExcel

The ConnectToExcel method declares and uses a connection string, database connection and database command in order to connect to the excel workbook.  The ReadExcel method uses the open connection along with a DataReader to read and loop through the data in the excel sheet.  Finally, UpdateExcel is used to make updates to the worksheet using the command object.

4. Declare a New Instance of the Object

The plain vanilla way to instantiate an object:

ExcelWorker builder = new ExcelWorker();

The second way:

ExcelWorker builder;

builder = new ExcelWorker();

And of course the third way which initializes the class fields as mentioned previously:

ExcelWorker builder = new ExcelWorker() {CustomerId = 123, CustomerName = "Bob"};

5. Static Classes and Methods

If the entire class is declared as static - public static int Counter(int theNumber)

 - then it will not be necessary to declare a new class object.  Instead the class and its methods, which also must be static, can be referenced directly in the code.   In this example, the ReadExcel method of the ExcelWorker class is called directly and the int 456 is passed to it.

private void button1_Click(object sender, EventArgs e)

       

 {

           

            ExcelWorker.ReadExcel(456);

        }

Also, if only the ReadExcel method is declared as static, then it can be referenced directly in the code but the other methods, ConnectToExcel and UpdateExcel, if not static, must be referenced by creating a new instance of the class.

class ExcelWorker

    {

        public int customerId;

        public string customerName;

        public static int ReadExcel(int theNumber)

        {

            return 123 + theNumber;

        }

         

    }

private void button1_Click(object sender, EventArgs e)

        {

           

            ExcelWorker.ReadExcel(456);

        }

So, Why Not Make All Methods Static?

If you declare multiple instances of a class, then each object will be able to keep up with distinct data via the class fields.  For example:  I might need two instances of the ExcelWorker class bulderOne and builderTwo.  The first object might need a string, customerName value of “Bob” while the second is named “Joe”.  If the class is static, it would not maintain separate values for the objects since two separate objects do not exist.

Summary

Class diagrams are useful in capturing the “mental model” of a class and mapping out its desired functionality through the detail of necessary fields and methods.  Although it is not always clear up front what methods and fields will be needed, this pre-planning process can help flesh out the needed functionality for the class and it may reveal some caveats in the model.  It should lead to more robust, quality code and time savings down the road.