Writing Text with a StreamWriter in C#

Encode QR in C# Writing Text with a StreamWriter

The File class offers a CreateText method, which takes the path to the file to create (either relative or absolute, as usual), and creates it for you if it doesn t already exist. If the file is already present, this method overwrites it. Unlike the WriteAllText method, it doesn t write any data initially the newly created file will be empty at first. The method returns an instance of the StreamWriter class, which allows you to write to the file. Example 11-21 shows the code we need to use that.

private static void CreateFile(string fullPath, string p) { using (StreamWriter writer = File.CreateText(fullPath)) { // Use the stream writer here } }

We re no longer writing the whole file in one big lump, so we need to let the StreamWriter know when we re done. To make life easier for us, StreamWriter implements IDisposable, and closes the underlying file if Dispose is called. This means that we can wrap it in a using block, as Example 11-21 shows, and we can be assured that it will be closed even if an exception is thrown.

So, what is a StreamWriter The first thing to note is that even though this chapter has Stream in the title, this isn t actually a Stream; it s a wrapper around a Stream. It derives from a class called TextWriter, which, as you might guess, is a base for types which write text into things, and a StreamWriter is a TextWriter that writes text into a Stream. TextWriter defines lots of overloads of Write and WriteLine methods, very similar to those we ve been using on Console in all of our examples so far.

If it is so similar in signature, why doesn t Console derive from Text Writer TextWriter is intended to be used with some underlying resource that needs proper lifetime management, so it implements IDisposable. Our code would be much less readable if we had to wrap every call on Console with a using block, or remember to call Dispose especially as it isn t really necessary. So, why make TextWriter implement IDisposa ble We do that so that our text-writing code can be implemented in terms of this base class, without needing to know exactly what sort of TextWriter we re talking to, and still handle the cleanup properly.

The File class s CreateText method calls a constructor on StreamWriter which opens the newly created file, and makes it ready for us to write; something like this:

return new StreamWriter(fullPath, false);

There s nothing to stop you from doing this yourself by hand, and there are many situations where you might want to do so; but the helper methods on File tend to make your code smaller, and more readable, so you should consider using those first. We ll look at using Stream Writer (and its partner, StreamReader) in this way later in the chapter, when we re dealing with different sorts of underlying streams.

Hang on, though. We ve snuck a second parameter into that constructor. What does that Boolean mean When you create a StreamWriter, you can choose to overwrite any existing file content (the default), or append to what is already there. The second Boolean parameter to the constructor controls that behavior. As it happen, passing false here means we want to overwrite.

This is a great example of why it s better to define nicely named enumerations, rather than controlling this sort of thing with a bool. If the value had not been false, but some mythical value such as OpenBehav ior.Overwrite, we probably wouldn t have needed to explain what it did. C# 4.0 added the ability to use argument names when calling methods, so we could have written new StreamWriter(fullPath, append: false), which improves matters slightly, but doesn t help you when you come across code that hasn t bothered to do that.

So, now we can easily complete the implementation of our CreateFile method, as shown in Example 11-22.

private static void CreateFile(string fullPath, string p) { using (StreamWriter writer = File.CreateText(fullPath)) { writer.Write(p); } }

We just write the string we ve been provided to the file. In this particular application, Example 11-22 isn t an improvement on Example 11-20 we re just writing a single string, so WriteAllText was a better fit. But StreamWriter is an important technique for less trivial scenarios.