Can we have Class.Empty Object like we have in String.Empty

Question

Hello,

I am using .NET 3.5. I am creating a simple class and wants to make sure that the class after processing should not be null or should not be a new too..

So if we can just Test it like that

Dim objClass as new Class()

' do some processing with the class '
' and then.. check that if this object returned is not empty '

If (objClass = Class.Empty) Then
//Do stuff
Else
//Do Other Stuff
End If

is there a way we can create this empty Field like we have in String.Empty?

Answer 1

Yes, it is called null for reference types, and new T() for value types.

Answer 2

there is design pattern Null Object pattern for it.

Answer 3

But in your example the class isn't technically empty. You've created an instance of Class using the default constructor. Who knows, the default constructor may have initialised over 10MB of string content. What your code is technically checking is, is the class equal to it's default state just after being constructed.

See this for example of VB constructors and what is happening.

If you correctly implemented CompareTo(...) you could call

If (objClass.CompareTo(new Class()) == 0) Then 
 //Do stuff 
Else
 //Do Other Stuff 
End If 

But that would seem overkill / expensive, but the only way it would work.

Another option would be: (sorry c# based example)

Interface IEmpyClass
{
   bool IsEmptyClass{get;}
}

public class Class : IEmptyClass
{
   public bool IsEmptyClass{get; private set;}

   public Class()
   {
     IsEmptyClass = true;
   }

   public void DoSomething()
   {
       // Do something
       IsEmptyClass = false;
   }
}

You would be responsible for implementing the example and changing the property when the class state changed from "empty", but that would be quicker in code, etc. and could cope when clases have constructors with members. It could be checked just with

If (objClass.IsEmptyClass) Then 
 //Do stuff 
Else
 //Do Other Stuff 
End If 

Answer 4

You can have an Extension method called "Empty" on "Class" which takes one parameter of type var (if .net 4.0) or object (older version). The implementation of this extension method will check whether its a null or not initialised to the default state and return true or false.

Answer 5

OK Guys Thanks for your help.. I have reached here so far.. Working very good now.

Public Class cc
    Private _obj As Boolean = False
    Public _index As Integer = 0

    Public ReadOnly Property IsEmpty() As Boolean
        Get
            Return _obj
        End Get
    End Property

    Public Property Index() As Integer
        Get
            Return _index
        End Get
        Set(ByVal value As Integer)
            _obj = False
            _index = value //I need to set it to false for every property of that class.
        End Set
    End Property


    Sub New()
        _obj = True
    End Sub



    Public Shared Function Empty(ByVal obj As cc) As Boolean
        If Not (obj Is Nothing) Then
            If Not (obj.IsEmpty()) Then
                Return False
            End If
        End If
        Return True
    End Function
End Class

---

In Separate Class--------------------

        Dim Sc As New cc()
        //'Sc=Nothing
        Sc.Index=1

        If (cc.Empty(Sc)) Then
            Label1.Text = "Empty"
        Else
            Label1.Text ="Not Empty"
        End If

Answer 6

To define an Empty field you have to create a static readonly property in your class. The trouble if you have also to define the = operator on your class.

For example, if your class contains only one field of type string, you can define an = operator that check if the string fields of the two instances are equals.