C# - StreamWriter Examples

StreamWriter. This helpful C# class writes text data and files. It enables easy and efficient text output. It is one of the first choices for file output in C#.

StreamReader

File

Using statements. StreamWriter is best placed in a using-statement. This ensures it is removed from memory when no longer needed. The syntax is easy to use once it is familiar.

First example. We first declare and initialize a new StreamWriter instance in a using construct. Please note how the System.IO namespace is included at the top of the file.

namespace

Important The "using" keyword means different things in different places. Near StreamWriter, it controls low-level resource usage.

using

Here A new StreamWriter is initialized with the file name "important.txt". Three writes are done using StreamWriter.

Info The Write method does not append a newline. The WriteLine methods append a newline "\r\n" at each call.

using System.IO; using (StreamWriter writer = new StreamWriter("important.txt")) { writer.Write("Word "); writer.WriteLine("word 2"); writer.WriteLine("Line"); }

(Text is in "important.txt" file.) Word word 2 Line

Append text. It is easy to append text to a file with StreamWriter. The file is not erased, but just reopened and new text is added to the end. We also see how you can specify a full path.

Start A new StreamWriter is initialized and it opens "C:\log.txt" for appending. The second argument (true) specifies an append operation.

Next The first line is appended to the file. If it is empty, the file begins with that string.

Then The second using construct reopens the "C:\log.txt" file and appends another string to it.

Note If for some reason the file was deleted, the line will be added just the same.

using System.IO; // Write single line to new file. using (StreamWriter writer = new StreamWriter("C:\\log.txt", true)) { writer.WriteLine("Important data line 1"); } // Append line to the file. using (StreamWriter writer = new StreamWriter("C:\\log.txt", true)) { writer.WriteLine("Line 2"); }

(File "log.txt" contains these lines.) Important data line 1 Line 2

Loop. Often we need to write out an array or List of strings. A good solution is to put the loop inside the using statement. This avoids creating more than one file handle in Windows.

for

Tip We use var, which makes the syntax shorter but equivalent in functionality. The file "loop.txt" is opened only once for writing.

var

Detail The format string can be easily used with Write. You have to specify a substitution marker {0} in the first parameter.

string.Format

using System.IO; // Use var type which is shorter. using (var writer = new StreamWriter("loop.txt")) { // Loop through ten numbers. for (int i = 0; i < 10; i++) { // Write format string to file. writer.Write("{0:0.0} ", i); } }

(These numbers are in the file "loop.txt".) 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

String interpolation. This syntax can be used to directly insert variables into a string before it is written with Write or WriteLine. We use the "$" char before the string.

Note String interpolation is about as fast as the older format syntax—it is a good alternative.

String Interpolation

using System.IO; using (StreamWriter writer = new StreamWriter("C:\\programs\\file.txt")) { string animal = "cat"; int size = 12; // Use string interpolation syntax to make code clearer. writer.WriteLine($"The {animal} is {size} pounds."); }

The cat is 12 pounds.

Summary. With the using-statement and StreamWriter, we wrote text files. StreamWriter is an excellent class—it is useful for many C# developers.