Easiest way to read from and write to files

Use File.ReadAllText and File.WriteAllText.

MSDN example excerpt:

// Create a file to write to.
string createText = "Hello and Welcome" + Environment.NewLine;
File.WriteAllText(path, createText);

...

// Open the file to read from.
string readText = File.ReadAllText(path);

---

In addition to File.ReadAllText, File.ReadAllLines, and File.WriteAllText (and similar helpers from File class) shown in another answer you can use StreamWriter/StreamReader classes.

Writing a text file:

using(StreamWriter writetext = new StreamWriter("write.txt"))
{
    writetext.WriteLine("writing in text file");
}

Reading a text file:

using(StreamReader readtext = new StreamReader("readme.txt"))
{
   string readText = readtext.ReadLine();
}

Notes:

You can use readtext.Dispose() instead of using, but it will not close file/reader/writer in case of exceptions

Be aware that relative path is relative to current working directory. You may want to use/construct absolute path.

Missing using/Close is very common reason of "why data is not written to file".

---

FileStream fs = new FileStream(txtSourcePath.Text,FileMode.Open, FileAccess.Read);
using(StreamReader sr = new StreamReader(fs))
{
   using (StreamWriter sw = new StreamWriter(Destination))
   {
            sw.Writeline("Your text");
    }
}

---

using (var file = File.Create("pricequote.txt"))
{
    ...........                        
}

using (var file = File.OpenRead("pricequote.txt"))
{
    ..........
}

Simple, easy and also disposes/cleans up the object once you are done with it.

---

The easiest way to read from a file and write to a file:

//Read from a file
string something = File.ReadAllText("C:\\Rfile.txt");

//Write to a file
using (StreamWriter writer = new StreamWriter("Wfile.txt"))
{
    writer.WriteLine(something);
}

---

@AlexeiLevenkov pointed me at another "easiest way" namely the extension method. It takes just a little coding, then provides the absolute easiest way to read/write, plus it offers the flexibility to create variations according to your personal needs. Here is a complete example:

This defines the extension method on the string type. Note that the only thing that really matters is the function argument with extra keyword this, that makes it refer to the object that the method is attached to. The class name does not matter; the class and method must be declared static.

using System.IO;//File, Directory, Path

namespace Lib
{
    /// <summary>
    /// Handy string methods
    /// </summary>
    public static class Strings
    {
        /// <summary>
        /// Extension method to write the string Str to a file
        /// </summary>
        /// <param name="Str"></param>
        /// <param name="Filename"></param>
        public static void WriteToFile(this string Str, string Filename)
        {
            File.WriteAllText(Filename, Str);
            return;
        }

        // of course you could add other useful string methods...
    }//end class
}//end ns

This is how to use the string extension method, note that it refers automagically to the class Strings:

using Lib;//(extension) method(s) for string
namespace ConsoleApp_Sandbox
{
    class Program
    {
        static void Main(string[] args)
        {
            "Hello World!".WriteToFile(@"c:\temp\helloworld.txt");
            return;
        }

    }//end class
}//end ns

I would never have found this myself, but it works great, so I wanted to share this. Have fun!

---

These are the best and most commonly used methods for writing to and reading from files:

using System.IO;

File.AppendAllText(sFilePathAndName, sTextToWrite);//add text to existing file
File.WriteAllText(sFilePathAndName, sTextToWrite);//will overwrite the text in the existing file. If the file doesn't exist, it will create it. 
File.ReadAllText(sFilePathAndName);

The old way, which I was taught in college was to use stream reader/stream writer, but the File I/O methods are less clunky and require fewer lines of code. You can type in "File." in your IDE (make sure you include the System.IO import statement) and see all the methods available. Below are example methods for reading/writing strings to/from text files (.txt.) using a Windows Forms App.

Append text to an existing file:

private void AppendTextToExistingFile_Click(object sender, EventArgs e)
{
    string sTextToAppend = txtMainUserInput.Text;
    //first, check to make sure that the user entered something in the text box.
    if (sTextToAppend == "" || sTextToAppend == null)
    {MessageBox.Show("You did not enter any text. Please try again");}
    else
    {
        string sFilePathAndName = getFileNameFromUser();// opens the file dailog; user selects a file (.txt filter) and the method returns a path\filename.txt as string.
        if (sFilePathAndName == "" || sFilePathAndName == null)
        {
            //MessageBox.Show("You cancalled"); //DO NOTHING
        }
        else 
        {
            sTextToAppend = ("\r\n" + sTextToAppend);//create a new line for the new text
            File.AppendAllText(sFilePathAndName, sTextToAppend);
            string sFileNameOnly = sFilePathAndName.Substring(sFilePathAndName.LastIndexOf('\\') + 1);
            MessageBox.Show("Your new text has been appended to " + sFileNameOnly);
        }//end nested if/else
    }//end if/else

}//end method AppendTextToExistingFile_Click

Get file name from the user via file explorer/open file dialog (you will need this to select existing files).

private string getFileNameFromUser()//returns file path\name
{
    string sFileNameAndPath = "";
    OpenFileDialog fd = new OpenFileDialog();
    fd.Title = "Select file";
    fd.Filter = "TXT files|*.txt";
    fd.InitialDirectory = Environment.CurrentDirectory;
    if (fd.ShowDialog() == DialogResult.OK)
    {
        sFileNameAndPath = (fd.FileName.ToString());
    }
    return sFileNameAndPath;
}//end method getFileNameFromUser

Get text from an existing file:

private void btnGetTextFromExistingFile_Click(object sender, EventArgs e)
{
    string sFileNameAndPath = getFileNameFromUser();
    txtMainUserInput.Text = File.ReadAllText(sFileNameAndPath); //display the text
}

---

Or, if you are really about lines:

System.IO.File also contains a static method WriteAllLines, so you could do:

IList<string> myLines = new List<string>()
{
    "line1",
    "line2",
    "line3",
};

File.WriteAllLines("./foo", myLines);

---

It's good when reading to use the OpenFileDialog control to browse to any file you want to read. Find the code below:

Don't forget to add the following using statement to read files: using System.IO;

private void button1_Click(object sender, EventArgs e)
{
    if (openFileDialog1.ShowDialog() == DialogResult.OK)
    {
         textBox1.Text = File.ReadAllText(openFileDialog1.FileName);  
    }
}

To write files you can use the method File.WriteAllText.

---

     class Program
    { 
         public static void Main()
        { 
            //To write in a txt file
             File.WriteAllText("C:\\Users\\HP\\Desktop\\c#file.txt", "Hello and Welcome");

           //To Read from a txt file & print on console
             string  copyTxt = File.ReadAllText("C:\\Users\\HP\\Desktop\\c#file.txt");
             Console.Out.WriteLine("{0}",copyTxt);
        }      
    }

---

private void Form1_Load(object sender, EventArgs e)
    {
        //Write a file
        string text = "The text inside the file.";
        System.IO.File.WriteAllText("file_name.txt", text);

        //Read a file
        string read = System.IO.File.ReadAllText("file_name.txt");
        MessageBox.Show(read); //Display text in the file
    }

---

Reading from file

string filePath = @"YOUR PATH";
List<string> lines = File.ReadAllLines(filePath).ToList();

Writing to file

List<string> lines = new List<string>();
string a = "Something to be written"
lines.Add(a);
File.WriteAllLines(filePath, lines);

---

You're looking for the File, StreamWriter, and StreamReader classes.

---