Structure implementing an interface.vb

Public Interface IHasETN
    Property ETN As Integer
End Interface

Public Class Class1
    Implements IHasETN

    Public Property ETN As Integer Implements IHasETN.ETN
    '    Get
    '        Throw New NotImplementedException()
    '    End Get
    '    Set(value As Integer)
    '        Throw New NotImplementedException()
    '    End Set
    'End Property
End Class

Public Structure Struct1
    Implements IHasETN

    Public Property ETN As Integer Implements IHasETN.ETN
    '    Get
    '        Throw New NotImplementedException()
    '    End Get
    '    Set(value As Integer)
    '        Throw New NotImplementedException()
    '    End Set
    'End Property
End Structure

Public Module Consumer

    ' This function works fine if anArray is always an array of classes that implement the IHasETN interface,
    ' but a call that passes an array of structures will not compile.
    Public Function FindIndexOfETN(ByVal iETN As Integer, ByVal anArray As IHasETN()) As Integer
        Return Array.FindIndex(anArray, Function(o) o.ETN = iETN)
    End Function

    Public Sub DoStuffWithArraysOfETN()
        Dim myClass1Array(3) As Class1
        myClass1Array(0) = New Class1()
        myClass1Array(0).ETN = 42
        myClass1Array(1) = New Class1()
        myClass1Array(1).ETN = 45
        myClass1Array(2) = New Class1()
        myClass1Array(2).ETN = 48

        Dim classiArrayETNIdx = FindIndexOfETN(48, myClass1Array)
        Console.WriteLine("classiArrayETNIdx: {0}", classiArrayETNIdx)

        Dim myStruct1Array(3) As Struct1
        myStruct1Array(0) = New Struct1()
        myStruct1Array(0).ETN = 32
        myStruct1Array(1) = New Struct1()
        myStruct1Array(1).ETN = 35
        myStruct1Array(2) = New Struct1()
        myStruct1Array(2).ETN = 38

        'Covariance for interface implementation (i.e. myStruct1Array "is a" IHasETN()) does not work for a structure (value type).
        'See https://stackoverflow.com/a/12677025/5652483
        'Dim structiArrayETNIdx = FindIndexOfETN(48, myStruct1Array) ' --> error BC30311: Value of type 'Struct1()' cannot be converted to 'IHasETN()'.
        'Console.WriteLine("structiArrayETNIdx: {0}", structiArrayETNIdx)
    End Sub

End Module