public interface I
{
    void Work();
}

public class A : I
{
    public void Work(){}
}

public class B : I
{
    public void Work(){}
}

public class C : I
{
    public void Work(){}
}

public class D : I
{
    public void Work(){}
}

public class E
{
    public static I CreateInstance()
    {
        switch(条件)
        {
            case 条件1:
                return new A();
            case 条件2:
                return new B();
            case 条件3:
                return new C();
            case 条件4:
                return new D();
            default:
                return null;
        }
    }
}

public class F
{
    I _instance;
    
    public F()
    {
        _instance = E.CreateInstance();
    }
    
    public void Work()
    {
        if(_instance != null)
        {
            _instance.Work();
        }
    }
}