定义抽象类MobileStorage，在里面写虚方法Read和Write，三个存储设备继承此抽象类，并重写Read和Write方法。Computer类中包含一个类型为MobileStorage的成员变量，并为其编写get/set器，这样Computer中只需要两个方法：ReadData和WriteData，并通过多态性实现不同移动设备的读写。

好的，我来回答你的问题。

首先，抽象类MobileStorage的定义如下：

public abstract class MobileStorage
{
    public abstract void Read();
    public abstract void Write();
}

然后，我们分别定义三个继承自MobileStorage的存储设备：

public class USBFlashDisk : MobileStorage
{
    public override void Read()
    {
        Console.WriteLine("USB flash disk reading data...");
    }

    public override void Write()
    {
        Console.WriteLine("USB flash disk writing data...");
    }
}

public class MemoryCard : MobileStorage
{
    public override void Read()
    {
        Console.WriteLine("Memory card reading data...");
    }

    public override void Write()
    {
        Console.WriteLine("Memory card writing data...");
    }
}

public class ExternalHardDisk : MobileStorage
{
    public override void Read()
    {
        Console.WriteLine("External hard disk reading data...");
    }

    public override void Write()
    {
        Console.WriteLine("External hard disk writing data...");
    }
}

接下来，我们在Computer类中添加一个类型为MobileStorage的成员变量，并编写get/set器：

public class Computer
{
    private MobileStorage storageDevice;

    public MobileStorage StorageDevice
    {
        get { return storageDevice; }
        set { storageDevice = value; }
    }

    public void ReadData()
    {
        storageDevice.Read();
    }

    public void WriteData()
    {
        storageDevice.Write();
    }
}

现在，我们就可以通过多态性实现不同移动设备的读写了，示例代码如下：

Computer computer = new Computer();
MobileStorage usb = new USBFlashDisk();
MobileStorage memoryCard = new MemoryCard();
MobileStorage externalHD = new ExternalHardDisk();

computer.StorageDevice = usb;
computer.ReadData();
computer.WriteData();

computer.StorageDevice = memoryCard;
computer.ReadData();
computer.WriteData();

computer.StorageDevice = externalHD;
computer.ReadData();
computer.WriteData();

输出结果为：

USB flash disk reading data...
USB flash disk writing data...
Memory card reading data...
Memory card writing data...
External hard disk reading data...
External hard disk writing data...

希望我的回答能够帮助到你！