Namespace CustomControlLibrary

    Public Class ButtonWithDesignTime
        Inherits System.Windows.Controls.Button

        Public Sub New()

            If System.ComponentModel.DesignerProperties.GetIsInDesignMode(Me) Then
                Me.Content = "This button is in design mode."
            End If
        End Sub
    End Class
End Namespace

namespace CustomControlLibrary
{
    public class ButtonWithDesignTime : System.Windows.Controls.Button
    {
        public ButtonWithDesignTime()
        {
            if (System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
            {
                this.Content = "This button is in design mode.";
            }
        }
    }
}