1.2.12 Defining an Additional Field of a Custom Type

A Class Can Contain More Than One Custom-Type Field

You already know that a field can use another class as its type.

For example, a Player class might contain:

public Team team;

The type is Team. The field name is team.

A class can contain additional fields whose types are also custom classes when the model requires more relationships.

Start from the Model

Do not add a custom-type field just because the syntax is available.

The field should represent a relationship required by the current design.

Suppose a soccer Match needs references to two teams:

Match
----------------
homeTeam : Team
awayTeam : Team

The corresponding C# fields can be:

public Team homeTeam;
public Team awayTeam;

Read Each Declaration as Type Then Name

For:

public Team homeTeam;

read:

Activity diagram showing Team field type, then homeTeam field name. — TeamFieldType.

For:

public Team awayTeam;

read:

Activity diagram showing Team field type, then awayTeam field name. — TeamFieldType.

Both fields can refer to Team objects, but they represent different roles in the Match.

Custom-Type Fields Store References

A custom-type field does not contain a complete duplicate of another object.

It stores a reference that can point to an object of the declared type.

Conceptually:

match1 : Match
   |
   +-- homeTeam ──> team1 : Team
   |
   +-- awayTeam ──> team2 : Team

The Match object can therefore be connected to two different Team objects.

The Field Name Describes the Relationship

The type tells you what kind of object can be referenced.

The field name tells you the role that object plays.

Compare:

public Team homeTeam;
public Team awayTeam;

Both fields have the same type, but the names make the two relationships distinct.

This is especially useful when a class needs more than one reference to objects of the same custom type.

Different Custom Types Can Also Appear Together

A class can use several custom types when the model requires them.

For example:

public Team homeTeam;
public Team awayTeam;
public Ball ball;

Here the Match can reference Team objects and a Ball object.

Each field should correspond to information or a relationship in the class design.

The Field Exists Before It Is Connected

Defining:

public Team homeTeam;

creates the field as part of the Match structure.

It does not automatically create a Team object or connect one to the field.

Until a reference is assigned, the field can contain:

null

Creating objects and assigning references are separate operations.

Keep Type Compatibility in Mind

A field declared as:

public Team homeTeam;

is intended to reference a Team object.

A field declared as:

public Ball ball;

is intended to reference a Ball object.

The type is part of the class design and controls which kind of object the field can reference.

Translate the UML Structure Directly

If the class diagram shows:

Match
----------------
homeTeam : Team
awayTeam : Team
ball : Ball

then the matching C# structure is:

public class Match
{
    public Team homeTeam;
    public Team awayTeam;
    public Ball ball;
}

The class diagram and the C# source are two representations of the same intended structure.

The Core Idea

When a class needs another relationship, define another field whose type matches the related class and whose name explains that relationship.

Activity diagram showing custom class type, then field declaration, then reference role in the model. — CustomClassType.

Use the model to decide which custom-type fields belong in the class.