Property

In VB.NET, a Property is similar to a Function. With a getter and a setter, it controls access to a value. This value is called a backing store.

Property notes

With Get, a property returns a value. With Set it stores a value. We must have both Get and Set unless we specify ReadOnly or WriteOnly on the property.

First example

On the Number property, we provide Get and Set blocks. In Get we return a value—the backing store count. In Set we receive a parameter and then store it in the count field.

Step 1 When the value 1 is assigned to the Number property, Set is executed. The count field stores the value 1.

Step 2 When the Number property is accessed but not assigned to, Get is executed. The value of the count field is returned.

Tip Additional logic can be inserted in either Get or Set. This extra layer of indirection is often used to validate arguments.

Class Example
    Private _count As Integer

    Public Property Number() As Integer
        Get
            Return _count
        End Get
        Set(ByVal value As Integer)
            _count = value
        End Set
    End Property
End Class

Module Module1
    Sub Main()
        Dim item As Example = New Example()

        ' Step 1: set property.
        item.Number = 4

        ' Step 2: get property.
        Console.WriteLine("NUMBER: {0}", item.Number)
    End Sub
End Module
NUMBER: 4

Provide Get, Set

A property must have both Get and Set members. It is possible to use the ReadOnly and WriteOnly keywords to eliminate this requirement.

Module Module1
    Property Answer() As Integer
        Get
            Return 42
        End Get
    End Property

    Sub Main()
        ' Program won't compile.
    End Sub
End ModuleError BC30124
Property without a 'ReadOnly' or 'WriteOnly' specifier must provide both a 'Get' and a 'Set'.

`ReadOnly`

Some properties are not meant to be assigned. For example, "Count" on collections is not mutable. The ReadOnly modifier changes the Property type to have only a Get method.

Detail The Count() property returns a constant Integer. But Get could perform any calculation or return the value of a field.

Warning If we try to assign a value to Count, we get this error: "Property Count is ReadOnly." So don't do that.

Class Example
    Public ReadOnly Property Count() As Integer
        Get
            Return 500
        End Get
    End Property
End Class

Module Module1
    Sub Main()
        Dim e As Example = New Example()
        Console.WriteLine(e.Count)
    End Sub
End Module
500

`WriteOnly`

Here we use the WriteOnly keyword on a property. WriteOnly means that a Property has only a Set() method and no Get method. In Main we create and assign to the Id property.

Tip In program design, WriteOnly properties may be confusing. Often a method is a clearer way to set values.

Class Example
    Dim _id As Integer

    Public WriteOnly Property Id
        Set(value)
            ' Sets the field from an external call.
            _id = value
        End Set
    End Property

    Public Sub Display()
        Console.WriteLine(_id)
    End Sub
End Class

Module Module1
    Sub Main()
        ' Create Example and assign Id.
        Dim e As Example = New Example
        e.Id = 100
        e.Display()
    End Sub
End Module
100

Automatic

Auto-implemented properties use a shorter syntax form. With them, we do not specify the Get or Set methods. We just the Property keyword and the declaration statement.

Detail This is a String property. We make it Public. It contains the name of the Dog objects created.

Detail This contains the weight of the dog. We can assign it, but it has a default value of 10.

Class Dog
    Public Property Name As String
    Public Property Weight As Integer = 10
End Class

Module Module1

    Sub Main()
        ' Use automatically-implemented properties.
        Dim dog As Dog = New Dog()
        dog.Name = "Old Yeller"
        dog.Weight = 50

        ' Print values.
        Console.WriteLine(dog.Name)
        Console.WriteLine(dog.Weight)
    End Sub

End ModuleOld Yeller
50

Limitations

The automatic property syntax in VB.NET is limited. We cannot use this syntax form if we want to insert code in the getter or setter. We must expand the property to do this.

Performance, methods

Properties are not magic. Instead they are implemented in the IL by generated methods. These have the property name prefixed with "get_" or "set_."

Note Properties can be inlined by the JIT. For setters and getters, a property will not normally cause any performance loss.

`Insert`

The Visual Studio editor provides some shortcuts for inserting properties. Try typing "pro" and pressing tab twice. Some code for a property will appear.

Tip To change the fields, please tab to them. Then enter the desired identifier or type.

A summary

Properties simplify syntax of VB.NET programs. And they help the language enforce consistency. We avoid writing Get() and Set() methods. Instead we use properties.