create dll in dotNet and use it in VB6

hi

i need to use a code written written dotNet  in vb6

i have created a classLibrary Project in dotNet  and i have built it , i got a dll file , a tlb file and an xml file

in vb6 , when i try to add the dll file as a reference i get this error message  : "unable to add reference to the specified file ..."

i added a reference to the tlb file and wrote a piece of code to call my function  "ConvertBMP" , i got a message at run time : "activex cannot create object..."

can anyone help me on that , is this the right way to do it or there is an other way ??

thanks in advance
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dotNet
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Imports System.IO.Path
Imports System.Drawing
Imports System.Drawing.Imaging
Public Class ClsConvertBMP
    Public Function ConvertBMP(ByVal BMPFullPath As String, _
                               ByVal NewNameImage As String, _
                               ByVal NewFormat As String) As Boolean
        Dim bAns As Boolean
        Dim sNewFile As String
        Dim imgFormat As ImageFormat = ImageFormat.Bmp
        Try
            'bitmap class in system.drawing.imaging
            Dim objBmp As New Bitmap(BMPFullPath)
            'below 2 functions in system.io.path
            sNewFile = GetDirectoryName(BMPFullPath)
            sNewFile += NewNameImage
            Select Case NewFormat
                Case "BMP"
                    imgFormat = ImageFormat.Bmp
                Case "GIF"
                    imgFormat = ImageFormat.Gif
                Case "JPG"
                    imgFormat = ImageFormat.Jpeg
                Case "TIF"
                    imgFormat = ImageFormat.Tiff
                Case "WMF"
                    imgFormat = ImageFormat.Wmf
            End Select
            sNewFile += "." & imgFormat.ToString
            objBmp.Save(sNewFile, imgFormat)
            bAns = True 'return TRUE on success
        Catch ex As Exception
            bAns = False 'return FALSE on echec
            'MsgBox("ConvertBMP : " & ex.Message)
        End Try
        Return bAns
    End Function
End Class
=========================================
VB
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Dim aa As New ClassLibrary1.ClsConvertBMP
Dim Ret As Boolean
Ret = aa.convertBMP("D:\test1.bmp", "Convert", "BMP")   <--- error here

End Sub
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