StreamWriter

The StreamWriter class in C# writes text data to files. It enables easy and efficient text output, and is a good first choice for file output in C#.

For reliability, StreamWriter is best placed in a using-statement. This ensures underlying system resources are removed from memory when no longer needed. The syntax is easy to use.

First example

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

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.

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. To do this, we can put a for-loop inside the using-statement—this avoids creating more than one file handle in Windows.

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.

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.

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