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How to: Write to a Text File (C# Programming Guide)

These examples show various ways to write text to a file. The first two examples use static methods on the System.IO.File class to write either a complete array of strings or a complete string to a text file. Example #3 shows how to add text to a file when you have to process each line individually before writing to the file. Examples 1-3 all overwrite all existing content in the file. Example #4 shows how to append text to an existing file.

class WriteTextFile
{
    static void Main()
    {

        // These examples assume a "C:\Users\Public\TestFolder" folder on your machine.
        // You can modify the path if necessary. 

        // Example #1: Write an array of strings to a file. 
        // Create a string array that consists of three lines. 
        string[] lines = {"First line", "Second line", "Third line"};
        System.IO.File.WriteAllLines(@"C:\Users\Public\TestFolder\WriteLines.txt", lines);


        // Example #2: Write one string to a text file. 
        string text = "A class is the most powerful data type in C#. Like structures, " +
                       "a class defines the data and behavior of the data type. ";
        System.IO.File.WriteAllText(@"C:\Users\Public\TestFolder\WriteText.txt", text);

        // Example #3: Write only some strings in an array to a file. 
        using (System.IO.StreamWriter file = new System.IO.StreamWriter(@"C:\Users\Public\TestFolder\WriteLines2.txt"))
        {
            foreach (string line in lines)
            {
                if (line.Contains("Second") == false)
                {
                    file.WriteLine(line);
                }
            }
        }

        // Example #4: Append new text to an existing file 
        using (System.IO.StreamWriter file = new System.IO.StreamWriter(@"C:\Users\Public\TestFolder\WriteLines2.txt", true))
        {
            file.WriteLine("Fourth line");
        }  
    }
}
/* Output (to WriteLines.txt):
    First line
    Second line
    Third line

 Output (to WriteText.txt):
    A class is the most powerful data type in C#. Like structures, a class defines the data and behavior of the data type.

 Output to WriteLines2.txt after Example #3:
    First line
    Third line

 Output to WriteLines2.txt after Example #4:
    First line
    Third line
    Fourth line
 */

Copy the code into a console application.

Replace "c:\testdir" with an actual folder name on your computer, or create a folder by that name.

The following conditions may cause an exception:

  • The file exists and is read-only.

  • The path name may be too long.

  • The disk may be full.

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