C# FileStream

last modified January 4, 2023

C# FileStream tutorial shows how to read & write files in C# with FileStream.

C# FileStream

FileStream provides a Stream for a file, supporting both synchronous and asynchronous read and write operations. A stream is a flow of data from a source into a destination. The source or destination can be a disk, memory, socket, or other programs.

When we use FileStream, we work with bytes. For more convenient work with text data, we can use StreamWriter and StreamReader.

C# FileStream write text

In the following example, we write text data into a file with FileStream.

using System.Text;

var fileName = @"C:\Users\Jano\Documents\words.txt";

using FileStream fs = File.OpenWrite(fileName);

var data = "falcon\nhawk\nforest\ncloud\nsky";
byte[] bytes = Encoding.UTF8.GetBytes(data);

fs.Write(bytes, 0, bytes.Length);

The example writes a couple of words into a text file.

using FileStream fs = File.OpenWrite(fileName);

The File.OpenWrite method opens a FileStream in a writing mode.

var data = "falcon\nhawk\nforest\ncloud\nsky";
byte[] bytes = Encoding.UTF8.GetBytes(data);

We have text data which we transform into bytes with Encoding.UTF8.GetBytes.

fs.Write(bytes, 0, bytes.Length);

The bytes are written to the FileStream with Write.

C# FileStream write text with StreamWriter

In the following example, we use FileStream in combination with StreamWriter.

var fileName = @"C:\Users\Jano\Documents\words.txt";

using FileStream fs = File.Create(fileName);
using var sr = new StreamWriter(fs);

sr.WriteLine("coin\nfalcon\nhawk\nforest");

Console.WriteLine("done");

The example writes text data into a file. For convenience, we use the StreamWriter, which writes characters to a stream in a particular encoding.

using FileStream fs = File.Create(fileName);
using var sr = new StreamWriter(fs);

A StreamWriter is created; it takes the FileStream as a parameter.

sr.WriteLine("coin\nfalcon\nhawk\nforest");

A line of text is written to the FileStream with WriteLine.

C# FileStream read text

In the following example, we read data from a text file with FileStream.

using System.Text;

var fileName = @"C:\Users\Jano\Documents\words.txt";

using FileStream fs = File.OpenRead(fileName);

byte[] buf = new byte[1024];
int c;

while ((c = fs.Read(buf, 0, buf.Length)) > 0)
{
    Console.WriteLine(Encoding.UTF8.GetString(buf, 0, c));
}

The example reads a text file and prints its contents. We read the data as bytes, transform them into strings using UTF8 encoding and finally, write the strings to the console.

using FileStream fs = File.OpenRead(fileName);

With File.OpenRead we open a file for reading. The method returns a FileStream.

byte[] buf = new byte[1024];

The buf is a byte array into which we read the data from the file.

while ((c = fs.Read(buf, 0, buf.Length)) > 0)
{
    Console.WriteLine(Encoding.UTF8.GetString(buf, 0, c));
}

The FileStream's Read method reads a block of bytes from the stream and writes the data in a given buffer. The first argument is the byte offset in array at which the read bytes will be placed. The second is the maximum number of bytes to read. The Encoding.UTF8.GetString decodes all the bytes in the specified byte array into a string.

C# FileStream read text with StreamReader

In the following example, we use FileStream in combination with StreamReader.

var fileName = @"C:\Users\Jano\Documents\words.txt";

using FileStream fs = File.OpenRead(fileName);
using var sr = new StreamReader(fs);

string line;

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

In the example, we read a text file. When we use StreamReader, we do not need to do the decoding of bytes into characters.

using FileStream fs = File.OpenRead(fileName);
using var sr = new StreamReader(fs);

We pass the FileStream to the StreamReader. If we do not explicitly specify the encoding, the default UTF8 is used.

string line;

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

We read the data with the StreamReader's WriteLine method. It returns the next line from the input stream, or null if the end of the input stream is reached.

C# FileStream CopyTo

The CopyTo method reads the bytes from the current stream and writes them to another stream.

var fileName = "words.txt";
using var fs = new FileStream(fileName, FileMode.Open);

var fileName2 = "words_copy.txt";
using var fs2 = new FileStream(fileName2, FileMode.OpenOrCreate);

fs.CopyTo(fs2);

Console.WriteLine("File copied");

The examples copies a text file with CopyTo method.

C# FileStream download image

In the following example, we download a small image file.

using var httpClient = new HttpClient();
var url = "http://webcode.me/favicon.ico";
byte[] imageBytes = await httpClient.GetByteArrayAsync(url);

using var fs = new FileStream("favicon.ico", FileMode.Create);
fs.Write(imageBytes, 0, imageBytes.Length);

Console.WriteLine("Image downloaded");

The example uses the HttpClient to download a small image file. The image is retrieved as an array of bytes. The bytes are then written to a FileStream.

using var fs = new FileStream("favicon.ico", FileMode.Create);
fs.Write(imageBytes, 0, imageBytes.Length);

We create a new file for writing. The bytes are written to the newly created file with the Write method.

C# FileStream read image

In the next example, we read an image file. We output the file as hexadecimal data.

var fileName = @"C:\Users\Jano\Documents\favicon.ico";

using var fs = new FileStream(fileName, FileMode.Open);

int c;
int i = 0;

while ((c = fs.ReadByte()) != -1)
{

    Console.Write("{0:X2} ", c);
    i++;

    if (i % 10 == 0)
    {
        Console.WriteLine();
    }
}

The example outputs a small image in hexadecimal notation.

while ((c = fs.ReadByte()) != -1)
{

The ReadByte method reads a byte from the file and advances the read position one byte. It returns the byte, cast to an Int32, or -1 if the end of the stream has been reached. It is OK to read the image by one byte since we deal with a very small image.

Console.Write("{0:X2} ", c);

The {0:X2} outputs the bytes in hexadecimal notation.

if (i % 10 == 0)
{
    Console.WriteLine();
}

We write a new line character after ten columns.

C# FileStream streaming

Streaming is a method of transmitting of data in a continuous stream that can be processed by the receiving computer before the entire file has been completely sent.

using var httpClient = new HttpClient();

var url = "https://cdn.netbsd.org/pub/NetBSD/NetBSD-9.2/images/NetBSD-9.2-amd64-install.img.gz";

var fname = Path.GetFileName(url);

var resp = await httpClient.GetAsync(url,
    HttpCompletionOption.ResponseHeadersRead);
resp.EnsureSuccessStatusCode();

using Stream ms = await resp.Content.ReadAsStreamAsync();

using FileStream fs = File.Create(fname);
await ms.CopyToAsync(fs);

Console.WriteLine("file downloaded");

A NetBSD USB image is downloaded using streaming operation.

using var httpClient = new HttpClient();

HttpClient is used to create HTTP requests.

var resp = await httpClient.GetAsync(url,
    HttpCompletionOption.ResponseHeadersRead);

With the HttpCompletionOption.ResponseHeadersRead option the async operation should complete as soon as a response is available and headers are read. The content is not read yet. The method will just read the headers and returns the control back.

using Stream ms = await resp.Content.ReadAsStreamAsync();

The ReadAsStreamAsync methods erialize the HTTP content and return a stream that represents the content as an asynchronous operation.

using FileStream fs = File.Create(fname);

We create a file stream with File.Create; it creates or overwrites a file in the specified path.

await ms.CopyToAsync(fs);

The data is copied continuously to the file stream.

In this article we have used FileStream to read and write files.

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