How to Write to a file using StreamWriter?

Question

in my Wpf app I'm using a class Person (that is a base class), and that contains a virtual method SaveData(), and other class Client that inherits from Person. How to override method SaveData() and keeping data from base?

Class Person

public virtual void SaveData()
{
   string arqName = string.Format("Person{0}" + ".txt", Id);
   StreamWriter file = new StreamWriter(arqNome);
   file.WriteLine("ID: " + Id);
   file.WriteLine("DOB: " + dOB);
   file.WriteLine("Name: " + name);
   file.WriteLine("Age: " + age);
   file.Flush();
   file.Close();     
}

Class Client

public override void SaveData()
{
   base.SaveData();
   string arqName = string.Format("Person{0}" + ".txt", Id);
   StreamWriter file = new StreamWriter(arqNome);
   file.WriteLine("Cod: " + cod);
   file.WriteLine("Credits: " + credits);
   file.Flush();
   file.Close();
}

The override method in Client is indeed override others data as Name, Age, DOB... I need to mantains both in same file.

Answer 1

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);
}

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);
}

Answer 2

AlexDev's answer is correct. But if you can't alter Person code (and given that you store data per file) you can use 'append' flag to add data into existed file. But it's not the best idea:

StreamWriter file = new StreamWriter(arqNome, append: true);

Answer 3

You should split it into 2 methods: SaveData() and WriteData(StreamWriter file). SaveData creates the stream and then calls WriteData. Then you override only the WriteDate method, calling the base.