StreamWriter是流装饰器，因此您可以更好地实例化FileStream并传递它到StreamWriter构造函数。因此，您可以自定义它。追加模式打开文件并将指针移动到文件末尾，因此您要编写的下一个内容将被追加。并使用使用指令显式调用 Close（）：

Person class SaveData（ ）：

StreamWriter is stream decorator, so you better instantiate FileStream and pass it to the StreamWriter constructor. Thus you can customize it. Append mode opens file and moves pointer to the end of file, so the next thing you write will be appended. And use using directive insted of explicitly calling Close():
Person class SaveData():

using (var fileStream = new FileStream(String.Format("Person{0}.txt", Id), FileMode.OpenOrCreate))
using (var streamWriter = new StreamWriter(fileStream))
{
    streamWriter.WriteLine("ID: " + Id);
    streamWriter.WriteLine("DOB: " + dOB);
    streamWriter.WriteLine("Name: " + name);
    streamWriter.WriteLine("Age: " + age);
}

客户端类SaveData（）：

Client class SaveData():

base.SaveData();
using (var fileStream = new FileStream(String.Format("Person{0}.txt", Id), FileMode.Append))
using (var streamWriter = new StreamWriter(fileStream))
{
    streamWriter.WriteLine("Cod: " + cod);
    streamWriter.WriteLine("Credits: " + credits);
}