TheDeveloperBlog.com

Home | Contact Us

C-Sharp | Java | Python | Swift | GO | WPF | Ruby | Scala | F# | JavaScript | SQL | PHP | Angular | HTML

C# File Handling

These C# examples show ways to handle files. They use many methods from the System.IO namespace.

Files. In memory, objects cease to exist when a program ends.

But files exist until deletion. With files we persist data. We handle them with types in System.IO.

StreamReader. For text files, StreamReader and StreamWriter are often the most useful types. We use StreamReader in a using block, a special syntax form.

StreamReaderStreamReader: ReadLineStreamWriter

Tip: Syntax forms like the using-statement are useful. They allow automatic cleanup of resources.

Based on:

.NET 4.5

C# program that uses StreamReader, ReadLine

using System.IO;

class Program
{
    static void Main()
    {
	// Read every line in the file.
	using (StreamReader reader = new StreamReader("file.txt"))
	{
	    string line;
	    while ((line = reader.ReadLine()) != null)
	    {
		// Do something with the line.
		string[] parts = line.Split(',');
	    }
	}
    }
}

Path. Before any file can be opened, it must be addressed. File paths are complex. They include the volume, directory, name and extension.

Path

Directory. We can manipulate directories on the file system with System.IO. The Directory type, and its static methods, is necessary for this.

Directory

FileInfo. We can get information about a file from the file system with FileInfo. This does not load the entire file into memory. It just reads stored stats.

FileInfoFileInfo Length

HTML, XML. Some files have lots of brackets and tags. These are usually HTML or XML files. We could write custom methods for each program, but standardized approaches exist.

HTML, XML

ReadAllText. This program uses this method to load in the file "file.txt" on the C: volume. Then it prints the contents of the file. The data is now stored in a string object.

File.ReadAllText

Tip: ReadAllText is the easiest way to put a file into a string. It is part of the System.IO namespace.

C# program that uses ReadAllText

using System;
using System.IO;

class Program
{
    static void Main()
    {
	string file = File.ReadAllText("C:\\file.txt");
	Console.WriteLine(file);
    }
}

ReadAllLines. Here we read all the lines from a file and place them in an array. The code reads lines from "file.txt" and uses a foreach-loop on them. This is efficient code.

File.ReadAllLines

C# program that uses ReadAllLines

using System.IO;

class Program
{
    static void Main()
    {
	// Read in every line in specified file.
	// ... This will store all lines in an array in memory.
	string[] lines = File.ReadAllLines("file.txt");
	foreach (string line in lines)
	{
	    // Do something with the line.
	    if (line.Length > 80)
	    {
		// Important code.
	    }
	}
    }
}

Count lines. We count the number of lines in a file with few lines of code. The example here is a bit slow. But it works. It references the Length property.

Line Count

C# program that counts lines

using System.IO;

class Program
{
    static void Main()
    {
	// Another method of counting lines in a file.
	// ... This is not the most efficient way.
	// ... It counts empty lines.
	int lineCount = File.ReadAllLines("file.txt").Length;
    }
}

Query. Does a line containing a specific string exist in the file? Maybe we want to see if a name or location exists in a line in the file. We use LINQ to find any matching line.

Count

C# program that uses LINQ on file

using System.IO;
using System.Linq;

class Program
{
    static void Main()
    {
	// See if line exists in a file.
	// ... Use a query expression to count matching lines.
	// ... If one matches, the bool is set to true.
	bool exists = (from line in File.ReadAllLines("file.txt")
		       where line == "Some line match"
		       select line).Count() > 0;
    }
}

ReadLines. This method does not immediately read in every line. It instead reads lines only as they are needed. We use it in a foreach-loop.

File.ReadLinesForeach

WriteAllLines. We can write an array to a file. When we are done within-memory processing, we often need to write the data to disk.

C# program that writes array to file

using System.IO;

class Program
{
    static void Main()
    {
	// Write a string array to a file.
	string[] stringArray = new string[]
	{
	    "cat",
	    "dog",
	    "arrow"
	};
	File.WriteAllLines("file.txt", stringArray);
    }
}

Results: file.txt

cat
dog
arrow

WriteAllText. A simple method, File.WriteAllText receives two arguments. It receives the path of the output file, and the exact string contents of the text file.

C# that uses WriteAllText

using System.IO;

class Program
{
    static void Main()
    {
	File.WriteAllText("C:\\deves.txt",
	    "Dot Net Perls");
    }
}

AppendAllText. We could read in a file, append to that in memory, and then write it out completely again. That is slow. Its more efficient to use an append.

File.AppendAllText

ReadAllBytes. We use File.ReadAllBytes to read an image (a PNG) into memory. With this code, we could cache an image in memory. It outperforms reading the image in each time.

File.ReadAllBytesFile.WriteAllBytes: Compress

C# that caches binary file

static class ImageCache
{
    static byte[] _logoBytes;
    public static byte[] Logo
    {
	get
	{
	    // Returns logo image bytes.
	    if (_logoBytes == null)
	    {
		_logoBytes = File.ReadAllBytes("Logo.png");
	    }
	    return _logoBytes;
	}
    }
}

TextReader. The TextReader and TextWriter types form the base class that other, more useful types derive from. Usually they are not useful on their own.

TextReaderTextWriter

Binary. BinaryReader and BinaryWriter make reading or writing a binary file much easier. These types introduce a level of abstraction over the raw data.

BinaryReaderBinaryWriter

Actions. We copy, delete, rename or get time information about files. These actions are available through the File type and the FileInfo type.

File.CopyFile.DeleteFile.ExistsFile.GetLastWriteTimeUtcFile.MoveFile.OpenFile.Replace

Streams take many forms. Sometimes leaving a file on the disk would impact performance or stability in a negative way. In these cases, please consider MemoryStream.

StreamMemoryStreamBaseStream

Seek: We can seek to a specific location in a file with the Seek method. Seek is useful with large binary files.

Seek

WebClient. Not every file we want to use is local. A file may be remote. We may need to access the network to download a file from a server.

WebClient

Office. It is common to need to control Microsoft Excel with C# code. We introduce a fast approach. This material may be outdated, but it still helps on many systems.

ExcelWord

CSV files. These are text-based databases. With the System.IO namespace, we can read them into a C# program. Sadly the TextFieldParser is slow.

TextFieldParser: Parse CSVCSV: Separate Files

Equality. How can we tell if two files are exactly equal? Unfortunately, the file system's metadata is not sufficient. A method that compares each byte is effective.

File Equals

Performance. When we access a file in Windows, the operating system puts that file into a memory cache. We provide a benchmark of file system caches.

Access Files TogetherMemoryMappedFileCache LengthsUnderstand File Caches

Research. The performance of file handling is an important part of computer programming. Often, optimizing how files are used is the most effective way to make a program faster.

One of the most significant sources of inefficiency is unnecessary input/output (I/O).

Code Complete

We can build small and fast storage, or large and slow storage, but not storage that is both large and fast.

Compilers: Principles, Techniques and Tools

File handling is hard. Even with the helpful types provided in the .NET Framework, it is fraught with errors. We must account for disk errors and invalid data. Testing is essential.


Related Links

Adjectives Ado Ai Android Angular Antonyms Apache Articles Asp Autocad Automata Aws Azure Basic Binary Bitcoin Blockchain C Cassandra Change Coa Computer Control Cpp Create Creating C-Sharp Cyber Daa Data Dbms Deletion Devops Difference Discrete Es6 Ethical Examples Features Firebase Flutter Fs Git Go Hbase History Hive Hiveql How Html Idioms Insertion Installing Ios Java Joomla Js Kafka Kali Laravel Logical Machine Matlab Matrix Mongodb Mysql One Opencv Oracle Ordering Os Pandas Php Pig Pl Postgresql Powershell Prepositions Program Python React Ruby Scala Selecting Selenium Sentence Seo Sharepoint Software Spellings Spotting Spring Sql Sqlite Sqoop Svn Swift Synonyms Talend Testng Types Uml Unity Vbnet Verbal Webdriver What Wpf