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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. With Get it returns a value. With Set it stores a value.
This example program uses the Property keyword.
On the Number() property,
we provide a Get block
and a Set block. In Get we return a value—the backing store _count. In Set we receive a parameter and then store it in the _count field.
Types:A property can have any data type. It does not need to be an Integer. It can be a Class.
And:Additional logic can be inserted in either Get or Set. This extra layer of indirection is often used to validate arguments.
VB.NET program that uses property syntax 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 e As Example = New Example() ' Set property. e.Number = 1 ' Get property. Console.WriteLine(e.Number) End Sub End Module Output 1
When the value 1 is assigned to the Number property, Set is executed. The _count field stores the value 1. When the Number property is accessed but not assigned to, Get is executed. The value of the _count field is returned.
Tip:A property must have both Get and Set members. It is possible to use the ReadOnly or WriteOnly keywords to eliminate this requirement.
Some properties in the .NET Framework are not meant to be assigned. For example the Count property on collections is not mutable. The ReadOnly modifier changes the Property type to only have a Get method.
Count:The Count() property returns a constant Integer. But Get could perform any calculation or return the value of a field.
Caution:If we try to assign a value to Count, we get this error: "Property Count is ReadOnly." So don't do that.
VB.NET program that uses ReadOnly Property 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 Output 500
This class, called Example, uses 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.
VB.NET program that uses WriteOnly keyword 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 Output 100
Auto-implemented properties use a shorter syntax form.
we do not specify the Get
or Set methods. We just the Property keyword and the declaration statement. We can specify a default value.
Name:This is a String property. We make it Public. It contains the name of the Dog objects created.Strings
Weight:This contains the weight of the dog. We can assign it, but it has a default value of 10.
Program, auto-implemented properties: VB.NET 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 Module Output Old 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.
However, there are situations in which you cannot use an auto-implemented property and must instead use standard, or expanded, property syntax.Auto-Implemented Properties: MSDN
The Visual Studio editor provides some shortcuts for inserting properties. Try typing "pro" and pressing tab twice. Some code for a property will appear.
To change the fields,
please tab to them,
and type something.
Properties simplify syntax of VB.NET programs. And they provide a facility in the language that enforces consistency. We avoid writing Get() and Set() methods. Instead we use properties.