C#接口问题，分不多，新手求大佬

[huanyanfei5003]

namespace 接口
{
class Program
{
public interface fly
{
Dream dream();
}
public class Dream
{

}
public class People : fly
{
public Dream dream()
{
Dream a = new Dream();
Console.WriteLine("a");
return a;
}
}
public class Animal : fly
{
public Dream dream()
{
Dream a = new Dream();
Console.WriteLine("b");
return a;
}
}
public class FF
{
fly activeDoment = null;//这里定义了一个接口字段，sring 、int的赋值我明白，接口型字段我不明白什么意思，也不知道如何赋值
//string str =“123”；
//int num = 3；
public fly ActiveDocument
{
get { return activeDoment; }
set {
activeDoment = value;
}
}
}
public class OrderList<T>: LinkedList<T>
{
public T FirstValue
{
get
{
if (First == null)
return default(T);
else
return First.Value;
}
}
}
static void Main(string[] args)
{
OrderList<fly> documentHistory = new OrderList<fly>();
FF ff = new FF();

Dream newDream = new Dream();
newDream = ff.ActiveDocument.dream();//报错，未将对象引入实例，如何引入？
}
}
}

[fanruinet]

fly activeDoment = new People(); 或 fly activeDoment = new Animal(); 都可以。只要new出来的对象实现了fly接口就能赋值。