public class A<T> where T : Stream {
　public T strm;
　public A() {}
}

public class B : A<FileStream> {
　public B() {
　　strm = new FileStream("", FileMode.Open);
　}
}

public class C : A<MemoryStream> {
　public C() {
　　strm = new MemoryStream();
　}
}