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Field Guide

Properties

If fields are usually private, how do you access these from outside the class?

C# provides a convenient feature called Properties that we can use to give controlled access to things the objects of a class will know. Essentially, properties are a way of adding a member to a class that looks and acts like a public field to other code, but is not actually a public field.

Properties: Why, When, and How

When coding knowing responsibilities in a class you will add fields for any data that you will capture within the objects. As you do this, you can think about whether this knowledge needs to be shared with others outside the class. If you do want to share this knowledge, you can create a property to achieve this.

Properties also give you the ability to make these values read or write only outside the class. Each property will define instructions to be performed when you read and instructions to be performed when you write to the property. If you provide both of these you have a property that can be read and written to. If you only provide one, then the property will be read or write only depending on which part you code.

In C

The start of a property declaration looks exactly like a field declaration: an access modifier followed by a data type and a name. Then, within curly braces you can declare up to two parts of the property:

  1. A get, which contains code that is executed when the property is read.
  2. A set, which contains code that is executed when the property is assigned to.

Both parts are optional, but can only be used once each in a property. A property containing only a get part is called a read-only property, and a property containing only a set part is called a set-only property. A property containing both is just called a property, or can be called a read-write property. Most properties are read-write or read-only.

Because get is used when the property is read, it must include a return statement which returns a value matching the data type of the property. The set part of a property has access to a special variable, value, which has the same data type as the property and the value of the right-hand side of the assignment statement in which the property was used.

The code within a get or set can be as simple or as complicated as you like, and can call other code within the same class or in libraries. Many properties are written as “wrappers” around a private field, and are used to better control access to the data in that field. Other properties may instead construct and return new data when they are read.

Example

We can revisit our Greeting class again. Notice the _message is private, but we have added a Message property that allows code outside the Greeting class to read or write to a Message property.

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}");
    }


    /// <summary>
    /// Each Greeting has a message that can be read and updated.
    /// </summary>
    public string Message
    {
        get
        {
            return _message;
        }
        set
        {
            _message = value;
        }
    }
}