File and Stream I/O

Introduction#

Manages files.

Syntax#

• new System.IO.StreamWriter(string path)
• new System.IO.StreamWriter(string path, bool append)
• System.IO.StreamWriter.WriteLine(string text)
• System.IO.StreamWriter.WriteAsync(string text)
• System.IO.Stream.Close()
• System.IO.File.ReadAllText(string path)
• System.IO.File.ReadAllLines(string path)
• System.IO.File.ReadLines(string path)
• System.IO.File.WriteAllText(string path, string text)
• System.IO.File.WriteAllLines(string path, IEnumerable<string> contents)
• System.IO.File.Copy(string source, string dest)
• System.IO.File.Create(string path)
• System.IO.File.Delete(string path)
• System.IO.File.Move(string source, string dest)
• System.IO.Directory.GetFiles(string path)

Parameters#

• Always make sure to close Stream objects. This can be done with a using block as shown above or by manually calling myStream.Close().
• Make sure the current user has necessary permissions on the path you are trying to create the file.
• Verbatim strings should be used when declaring a path string that includes backslashes, like so: @"C:\MyFolder\MyFile.txt"

Reading from a file using the System.IO.File class

You can use the System.IO.File.ReadAllText function to read the entire contents of a file into a string.

string text = System.IO.File.ReadAllText(@"C:\MyFolder\MyTextFile.txt");

You can also read a file as an array of lines using the System.IO.File.ReadAllLines function:

string[] lines = System.IO.File.ReadAllLines(@"C:\MyFolder\MyTextFile.txt");

Writing lines to a file using the System.IO.StreamWriter class

The System.IO.StreamWriter class:

Implements a TextWriter for writing characters to a stream in a particular encoding.

Using the WriteLine method, you can write content line-by-line to a file.

Notice the use of the using keyword which makes sure the StreamWriter object is disposed as soon as it goes out of scope and thus the file is closed.

string[] lines = { "My first string", "My second string", "and even a third string" };
using (System.IO.StreamWriter sw = new System.IO.StreamWriter(@"C:\MyFolder\OutputText.txt"))
{
    foreach (string line in lines)
    {
        sw.WriteLine(line);
    }
}

Note that the StreamWriter can receive a second bool parameter in it’s constructor, allowing to Append to a file instead of overwriting the file:

bool appendExistingFile = true;
using (System.IO.StreamWriter sw = new System.IO.StreamWriter(@"C:\MyFolder\OutputText.txt", appendExistingFile ))
{
    sw.WriteLine("This line will be appended to the existing file");
}

Writing to a file using the System.IO.File class

You can use the System.IO.File.WriteAllText function to write a string to a file.

string text = "String that will be stored in the file";
System.IO.File.WriteAllText(@"C:\MyFolder\OutputFile.txt", text);

You can also use the System.IO.File.WriteAllLines function which receives an IEnumerable<String> as the second parameter (as opposed to a single string in the previous example). This lets you write content from an array of lines.

string[] lines = { "My first string", "My second string", "and even a third string" };
System.IO.File.WriteAllLines(@"C:\MyFolder\OutputFile.txt", lines);

Lazily reading a file line-by-line via an IEnumerable

When working with large files, you can use the System.IO.File.ReadLines method to read all lines from a file into an IEnumerable<string>. This is similar to System.IO.File.ReadAllLines, except that it doesn’t load the whole file into memory at once, making it more efficient when working with large files.

IEnumerable<string> AllLines = File.ReadLines("file_name.txt", Encoding.Default);

The second parameter of File.ReadLines is optional. You may use it when it is required to specify encoding.

It is important to note that calling ToArray, ToList or another similar function will force all of the lines to be loaded at once, meaning that the benefit of using ReadLines is nullified. It is best to enumerate over the IEnumerable using a foreach loop or LINQ if using this method.

Create File

File static class

By using Create method of the File static class we can create files. Method creates the file at the given path, at the same time it opens the file and gives us the FileStream of the file. Make sure you close the file after you are done with it.

ex1:

var fileStream1 = File.Create("samplePath");
/// you can write to the fileStream1
fileStream1.Close();

ex2:

using(var fileStream1 = File.Create("samplePath"))
{
    /// you can write to the fileStream1
}

ex3:

File.Create("samplePath").Close();

FileStream class

There are many overloads of this classes constructor which is actually well documented here. Below example is for the one that covers most used functionalities of this class.

var fileStream2 = new FileStream("samplePath", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None);

You can check the enums for FileMode, FileAccess, and FileShare from those links. What they basically means are as follows:

FileMode: Answers “Should file be created? opened? create if not exist then open?” kinda questions.

FileAccess: Answers “Should I be able to read the file, write to the file or both?” kinda questions.

FileShare: Answers “Should other users be able to read, write etc. to the file while I am using it simultaneously?” kinda questions.

Copy File

File static class

File static class can be easily used for this purpose.

File.Copy(@"sourcePath\abc.txt", @"destinationPath\abc.txt");
File.Copy(@"sourcePath\abc.txt", @"destinationPath\xyz.txt");

Remark: By this method, file is copied, meaning that it will be read from the source and then written to the destination path. This is a resource consuming process, it would take relative time to the file size, and can cause your program to freeze if you don’t utilize threads.

Move File

File static class

File static class can easily be used for this purpose.

File.Move(@"sourcePath\abc.txt", @"destinationPath\xyz.txt");

Remark1: Only changes the index of the file (if the file is moved in the same volume). This operation does not take relative time to the file size.

Remark2: Cannot override an existing file on destination path.

Delete File

string path = @"c:\path\to\file.txt";
File.Delete(path);

While Delete does not throw exception if file doesn’t exist, it will throw exception e.g. if specified path is invalid or caller does not have the required permissions. You should always wrap calls to Delete inside try-catch block and handle all expected exceptions. In case of possible race conditions, wrap logic inside lock statement.

Files and Directories

Get all files in Directory

 var FileSearchRes = Directory.GetFiles(@Path, "*.*", SearchOption.AllDirectories);

Returns an array of FileInfo, representing all the files in the specified directory.

Get Files with specific extension

 var FileSearchRes = Directory.GetFiles(@Path, "*.pdf", SearchOption.AllDirectories);

Returns an array of FileInfo, representing all the files in the specified directory with the specified extension.

Async write text to a file using StreamWriter