Can't inherit the class which is inherited DLL class

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VB

WPF

For some buttons based on the user, we used to set previlege.for that we have 2 functions.
Our requirement is, we need to convert the toolbar to DLL. we converted the toolbar except the database-dependent function addbutton, visible button in the below example.
so after conversion we inherited the dll in a class called 'BaseToolBar'.

Now my problem is i can't use the class 'Basetoolbar' in another form.it prompt the error like can't create instance. but instead of Dll inherited class, if we inherited normal class it's working.
Kindly provide any solution for that

Imports Toolbar

 Public Class BaseToolBar
     Inherits UcBaseToolBar
     Public Overrides Function AddButtons(ByVal ButtonNames() As String, Optional ByVal formname As String = "")

     End Function

     Public Overrides Function VisibleButtons(ByVal ButtonNames() As String, Optional ByVal formname As String = "", Optional ByVal mnuName As String = "")

     End Function

 End Class

In the above example,
1. Toolbar is the DLL we created
2.UcBaseToolBar is the usercontrol which is in DLL class.we inherited in 'BaseToolBar' class
3.Now where ever the tool bar used in forms , we need to replace with the class 'BaseToolBar' like the below

<usercontrol x:class="test" xmlns:x="#unknown">
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
                 xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
                 xmlns:y ="clr-namespace:Toolbar;assembly=Toolbar" 
                   xmlns:y1="clr-namespace:IT "
                 mc:Ignorable="d" 
                 d:DesignHeight="300" d:DesignWidth="300">
        <grid>
            <y1:basetoolbar xmlns:y1="#unknown"></y1:basetoolbar>
        </grid>
    </usercontrol>

this <y1:basetoolbar xmlns:y1="#unknown">line creates error


Thanks & Regards
jai