C# Properties

While accessor methods provide a powerful control mechanism in object-oriented languages, they also require a lot of typing the same code syntax over and over (we often call this boilerplate). Many languages therefore introduce a mechanism for quickly defining basic accessors. In C#, we have Properties. Let’s rewrite our Student class with Properties:

public class Student {

    private string _first;
    /// <summary>The student's first name</summary>
    public string First {
        get { return _first; }
        set { if(value.Length > 0) _first = value;}

    private string _last;
    /// <summary>The student's last name</summary>
    public string Last {
        get { return _last; }
        set { if(value.Length > 0) _last = value; }

    private uint _wid;
    /// <summary>The student's Wildcat ID number</summary>
    public uint Wid {
        get { return this._wid; }

    /// <summary>The student's full name</summary>
    public string FullName 
        return $"{First} {Last}"

    /// <summary>The student's nickname</summary>
    public string Nickname { get; set; }

    /// <summary>Constructs a new student object</summary>
    /// <param name="first">The new student's first name</param>
    /// <param name="last">The new student's last name</param>
    /// <param name="nick">The new student's nickname</param>
    /// <param wid="wid">The new student's Wildcat ID number</param>
    public Student(string first, string last, string nick, uint wid) {
        _first = first;
        _last = last;
        Nickname = nick;
        _wid = wid;

If you compare this example to the previous one, you will note that the code contained in bodies of the get and set are identical to the corresponding getter and setter methods. Essentially, C# properties are shorthand for writing out the accessor methods. In fact, when you compile a C# program it transforms the get and set back into methods, i.e. the get in first is used to generate a method named get_First().

While properties are methods, the syntax for working with them in code is identical to that of fields, i.e. if we were to create and then print a Student’s identifying information, we’d do something like:

Student willie = new Student("Willie", "Wildcat", "WillieCat", 99999999);
Console.Write("Hello, ")
Console.Write("Your WID is:");

Note too that we can declare properties with only a get or a set body, and that properties can be derived from other state rather than having a private backing field.


Properties are Methods

While C# properties are used like fields, i.e. Console.WriteLine(willie.Wid) or willie.First = "William", they are actually methods. As such, they do not add structure to hold state, hence the need for a backing variable.

The Nickname property in the example above is special syntax for an implicit backing field - the C# compiler creates the necessary space to hold the value. But we can only access the value stored through that property. If you need direct access to it, you must create a backing variable.

However, we don’t always need a backing variable for a Property getter if the value of a property can be calculated from the current state of the class, e.g., consider our FullName property in our Student class:

public string FullName 
            return $"{First} {Last}"

Here we’re effectively generating the value of the FullName property from the First and Last properties every time the FullName property is requested. This does cause a bit more computation, but we also know that it will always reflect the current state of the first and last names.

Auto-Property Syntax

Not all properties need to do extra logic in the get or set body. Consider our Vector3 class we discussed earlier. We used public fields to represent the X, Y, and Z components, i.e.:

public double X = 0;

If we wanted to switch to using properties, the X property would end up like this:

private double _x = 0;
public double X 
    return _x;
    _x = value;

Which seems like a lot more work for the same effect. To counter this perception and encourage programmers to use properties even in cases like this, C# also supports auto-property syntax. An auto-property is written like:

public double X {get; set;} = 0;

Note the addition of the {get; set;} - this is what tells the compiler we want a property and not a field. When compiled, this code is transformed into a full getter and setter whose bodies match the basic get and set in the example above. The compiler even creates a private backing field (but we cannot access it in our code, because it is only created at compile time). Any time you don’t need to do any additional logic in a get or set, you can use this syntax.

Note that in the example above, we set a default value of 0. You can omit setting a default value. You can also define a get-only autoproperty that always returns the default value (remember, you cannot access the compiler-generated backing field, so it can never be changed):

public double Pi {get;} = 3.14;

In practice, this is effectively a constant field, so consider carefully if it is more appropriate to use that instead:

public const PI = 3.14;

While it is possible to create a set-only auto-property, you will not be able access its value, so it is of limited use.

Expression-Bodied Members

Later versions of C# introduced a concise way of writing functions common to functional languages known as lambda syntax, which C# calls Expression-Bodied Members.

Properties can be written using this concise syntax. For example, our FullName get-only derived property in the Student written as an expression-bodied read-only property would be:

public FullName => $"{FirstName} {LastName}"

Note the use of the arrow formed by an equals and greater than symbol =>. Properties with both a getter and setter can also be written as expression-bodied properties. For example, our FirstName property could be rewritten:

public FirstName 
  get => _first;
  set => if(value.Length > 0) _first = value;

This syntax works well if your property bodies are a single expression. However, if you need multiple lines, you should use the regular property syntax instead (you can also mix and match, i.e. use an expression-bodied get with a regular set).

Different Access Levels

It is possible to declare your property as public and give a different access level to one of the accessors, i.e. if we wanted to add a GPA property to our student:

public double GPA { get; private set; } = 4.0;

In this case, we can access the value of the GPA outside of the student class, but we can only set it from code inside the class. This approach works with all ways of defining a property.

Init Property Accessor

C# 9.0 introduced a third accessor, init. This also sets the value of the property, but can only be used when the class is being initialized, and it can only be used once. This allows us to have some properties that are immutable (unable to be changed).

Our student example treats the Wid as immutable, but we can use the init keyword with an auto-property for a more concise representation:

public uint Wid {get; init;}

And in the constructor, replace setting the backing field (_wid = wid) with setting the property (Wid = wid). This approach is similar to the public property/private setter, but won’t allow the property to ever change once declared.