internal class MyBaseClass<T> : IMyInterface
{
    ....
    ....
    public void DoWork();
}

internal class Helper
{
    private IMyInterface m_member;
    
    public void DoSomething()
    {
        //here i want to call the DoWork method of the base class
        //that does not work because T is not known
        ((MyBaseClass<T>)m_member).DoWork();
    }
}

internal interface IMyExtendedInterface : IMyInterface
{
   void DoWork();
}

internal class MyBaseClass<T> : IMyExtendedInterface
{
   // wie gehabt
}

internal class Helper
{
    private IMyExtendedInterface m_member;

    public void DoSomething()
    {
        m_member.DoWork(); // Ganz ohne Cast und ohne die Gefahr von Laufzeitfehlern.
    }
}