C# Generics and polymorphism: an oxymoron?

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I just want to confirm what I've understood about Generics in C#. This has come up in a couple code bases I've worked in where a generic base class is used to create type-safe derived instances. A very simple example of what I'm talking about,

public class SomeClass<T>
{
    public virtual void SomeMethod(){ }
}

public class DeriveFrom :SomeClass<string>
{
    public override void SomeMethod()
    {
        base.SomeMethod();
    }
}

The problem comes up when I then want to use derived instances in a polymorphic way.

public class ClientCode
{
    public void DoSomethingClienty()
    {
        Factory factory = new Factory();
        //Doesn't compile because SomeClass needs a type parameter!
        SomeClass someInstance = factory.Create();

        someInstance.SomeMethod();
    }
}

It seems that once you introduce a Generic into an inheritance hierarchy or interface, you can no longer use that family of classes in a polymorphic way except perhaps internal to itself. Is that true?

polymorphism generics c#
6 Answers
public Class ReflectionReport<T> 
{
    // This class uses Reflection to produce column-based grids for reporting.  
    // However, you need a type in order to know what the columns are. 
}

... so, in another class you have...

public Class ReflectionReportProject {
    ReflectionReport<Thang> thangReport = new ReflectionReport<Thang>(); 
    ReflectionReport<Thong> thongReport = new ReflectionReport<Thong>(); 



    ... some more stuff ...

    // Now, you want to pass off all of the reports to be processed at one time....
    public ReflectionReport<????>[] ProvideReports() 
    {
        return new ReflectionReport<????>[] { thangReport, thongReport } ;
    }
}
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