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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.
Program that uses property syntax: VB.NET 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.
In this program, the Count() property returns a constant Integer. But Get could perform any calculation and return the value of a field. ReadOnly does not require a constant return value.
Caution:If we try to assign a value to Count, we get this error: "Property Count is ReadOnly." So don't do that.
Program that uses ReadOnly Property: VB.NET 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
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.