Knowing Responsibilities

Objects of a class can store data in fields, in the same way that structs contain fields. Fields are also called instance variables, because they are implemented as variables declared within the class, and allocated within each instance (object) created from that class.

Just like a variable, every field has a data type, an identifier, and a value. Most fields are variable, meaning their value can be changed. It is also possible for a field to be constant, in which case the value can only be set once.

As we saw with member functions, fields are accessible anywhere within the class where they are defined. That means a field can be accessed and modified (assuming it is not constant) within any of the methods in the same class. This is one of the concepts that makes classes powerful, as it allows us to share data between methods without needing to see these as parameters.

Knowing: Why, When, and How

When you think of a role, objects that play this role will need to know things in order to fulfil their responsibilities. These are your knowing responsibilities for the role and define the fields you would create within the class.

For example, an object-oriented version of the Fly Catch game would have Fly objects. These will know if they are on the web at the moment, where they are on the screen, when they appeared, and when they should escape. This knowledge will let the fly behave in a way that we want.

In C#

Hopefully the syntax for declaring a field looks familiar, because it is the same as the syntax for declaring a variable.

Typically, we declare our fields at the top of our class definition, and then initialise their values inside the class’s constructor(s).

Example

If we revisit our Greeting class, we can see that it has one _message field. This is coded as a string field within the class as shown below.

using static System.Console;

class Greeting
{
    /// <summary>
    /// Each Greeting knows its message, and can use this when it prints the greeting.
    /// </summary>
    private string _message;

    public Greeting(string message)
    {
        _message = message;
    }

    public void Print()
    {
        WriteLine(_message);
    }

    public void Greet(string name)
    {
        WriteLine($"Hello {name}! {_message}");
    }

    public string Message
    {
        get
        {
            return _message;
        }
        set
        {
            _message = value;
        }
    }
}