ReadOnly properties in Vb.Net

Question

Hi,

What is the difference between a property declared as ReadOnly and a property that has a public Get method but a private SET method? Are these two things not equivalent? If they are, then why do we have two ways of doing the same thing?

Regards,
Sougata Ghosh

Answer 1

They both achieve the same thing. You cannot set data in the property. I'll assume it does it internally with the Read-only attribute as opposed to making the Set Private or one doesn't have the Set in the property. Either way you want to do it, it's a read-only property.

Answer 2

Hi,
there are differences.

A value for the read-only property that you can set only once during instantiation (constructor or direct assignment to the property).

A value of the property with a private setter that you can set at any time during execution, but only within the class (object).

Try following console demo:

Module Module75
  Sub Main()
    Try
      Dim c As New Demo
      c.Execute()
    Catch ex As Exception
      Console.WriteLine(ex.ToString)
    End Try
    Console.WriteLine("Continue enter key")
    Console.ReadKey()
  End Sub
  Friend Class Demo
    Friend Sub Execute()
      Try
        Dim c As New DemoProp("Prop 1")
        c.SetPropValue("Prop 3")
        Console.WriteLine(c.ROProp1)
        Console.WriteLine(c.ROProp2)
        Console.WriteLine(c.PrivateSetterProp)
      Catch ex As Exception
        Console.WriteLine(ex.Message)
      End Try
    End Sub
  End Class

  ''' <summary>
  ''' Demo class with different properties
  ''' </summary>
  Public Class DemoProp

    ''' <summary>
    ''' Constructor
    ''' </summary>
    ''' <param name="parameter">value to initial set readonly property</param>
    Public Sub New(parameter As String)
      ROProp1 = parameter
    End Sub

    ''' <summary>
    ''' readonly property with value from constructor
    ''' </summary>
    ''' <returns>value assigned in constructor</returns>
    Public ReadOnly Property ROProp1 As String

    ''' <summary>
    ''' readonly property with assigned value 
    ''' </summary>
    ''' <returns>assigned value during instantiation</returns>
    Public ReadOnly Property ROProp2 As String = "Prop 2"

    ''' <summary>
    ''' backing field
    ''' </summary>
    Private _privatesetterProp As String
    ''' <summary>
    ''' property with private setter
    ''' </summary>
    ''' <returns>backing field</returns>
    Public Property PrivateSetterProp As String
      Get
        Return Me._privatesetterProp
      End Get
      Private Set(value As String)
        Me._privatesetterProp = value
      End Set
    End Property

    ''' <summary>
    ''' method to use property with private setter
    ''' </summary>
    ''' <param name="parameter">value to use private setter</param>
    Public Sub SetPropValue(parameter As String)
      PrivateSetterProp = parameter
    End Sub

  End Class

End Module