How to generate test method for a singleton class instance in Vb.Net

Question

The following is my code and I am using the Test Project available in Visual Studio 2008

Public Class Calculator
    Public Shared objCalculator As Calculator = Nothing
    Private Sub New()

    End Sub
    Public Shared Function GetInstance() As Calculator
        If objCalculator Is Nothing Then
            objCalculator = New Calculator
        End If
        Return objCalculator
    End Function
End Class

Test Method

 '''<summary>
    '''A test for GetInstance
    '''</summary>
    <TestMethod()> _
    Public Sub GetInstanceTest()
        Dim expected As Calculator = Nothing
        Dim actual As Calculator
        actual = Calculator.GetInstance
        Assert.AreNotEqual(expected, actual)
        Assert.Inconclusive("Verify the correctness of this test method.")
    End Sub

Am I following the correct way to test the Singleton class. I am using Assert.AreNotEqual(expected, actual) this,

Answer 1

While you are using Assert.AreNotEqual() correctly, the naming of expected is a bit strange. A better name is probably notExpected . Or just leave out the expected variable and pass in Nothing directly to Assert.AreNotEqual() .

Also, singletons are dangerous things. Your test will, when calling GetInstance , create a new Calculator . The next test that calls GetInstance will however not create a new Calculator , but reuse the one that the first test created. What you have done is that you have introduced a dependency between the tests. That's something you typically don't want to have. My advise is that you don't use singletons.