C# Stream Tutorial: StreamReader, StreamWriter with Example

Details: Last Updated: 28 October 2019

In C# file operations, normally streams are used to read and write to files. A stream is an additional layer created between an application and a file. The stream is used to ensure smooth read and write operations to the file.

Streams are normally used when reading data from large files. By using streams, the data from large files in broken down into small chunks and sent to the stream. These chunks of data can then be read from the application.

The reason for breaking it down into small chunks is because of the performance impact of reading a big file in one shot. If you were to read the data from say, a 100 MB file at one shot, your application could just hang and become unstable. The best approach is then to use streams to break the file down into manageable chunks.

So when a write operation is carried out on file, the data to be written, is first written to the stream. From the stream, the data is then written to the file. The same goes for the read operation. In the read operation, data is first transferred from the file to the stream. The data is then read from the application via the stream. Let's look at an example of how we can read and write using streams.

In this tutorial, you will learn-

Stream Reader
Stream Writer

Stream Reader

The stream reader is used to read data from a file using streams. The data from the file is first read into the stream. Thereafter the application reads the data from the stream.

For our example, we will assume that we have a file in the D drive called Example.txt. The file will be a simple text file and have 2 lines as shown below

Guru99 - .Net
Guru99 -C#

For our example, we will create a simple Console application and work with File streams

Let's look at an example of how we can use streams for reading data from a file. Enter the below code in the program.cs file.

using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DemoApplication
{
  class Tutorial
  {
   static void Main(string[] args)
   {
    String path = @"D:\Example.txt";

    using (StreamReader sr = File.OpenText(path))
    {
     String s = "";

     while ((s = sr.ReadLine()) != null)
     {
      Console.WriteLine(s);
     }
    }
   Console.ReadKey();
  }
 }
}

Code Explanation:-

First, we are declaring a stream reader object. The stream reader object is used in C# to define a stream from the file to the application. The data will be pushed from the file to the stream whenever data is read from the file. The File.OpenText is used to open the file "Example.txt" in read-only mode. The handler to the file is then sent to the stream reader object.
Next, we are defining a temporary variable 's' which will be used to read all the data from the file.
We then use the stream reader method ReadLine to read each line from the stream buffer. When we perform this operation, each line will be first transferred from the file to the buffer. Then the string line will be transferred from the buffer to the variable 's'. We then write the contents of the string 's' to the console.

When the above code is set, and the project is run using Visual Studio, you will get the below output.

Output:-

From the output, you can see that the Stream Reader read both the lines from the file. Finally, the lines of the string read from the stream were sent to the Console.

Stream Writer

The stream writer is used to write data to a file using streams. The data from the application is first written into the stream. After that the stream writes the data to the file. Let's look at an example of how we can use streams for writing data from a file. Enter the below code in the program.cs file.

using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DemoApplication
{
  class Tutorial
  {
   static void Main(string[] args)
   {
    String path = @"D:\Example.txt";
    
    using (StreamWriter sr = File.AppendText(path))
    {
     sr.WriteLine("Guru99 - ASP.Net");
     sr.Close();

     Console.WriteLine(File.ReadAllText(path));
    }
    Console.ReadKey();
  }
 }
}

Code Explanation:-

First, we are declaring a stream writer object. The stream writer object is used in C# to define a stream. The stream is then used to write data from the application to the file. The data will be pushed from the application to the stream whenever data needs to be written. The File.AppendText command is used to open the file "Example.txt" in an append mode. The handler to the file is then sent to the stream writer object.
We are using the stream write method Writeline to write the line "Guru99 – ASP.Net" to the stream. From the stream, the line will then be written to the file.
We then close the stream writer after writing to the file. It's normally a good practice to close file handlers when the file is no longer required for writing purposes.
Finally, we are reading the contents of the file again and writing it to the console log. This is to check as to whether the line was written to the file.

When the above code is set, and the project is run using Visual Studio, you will get the below output.

Output:-